Please note, that this guide is still a work in progress, mainly the how to beat X champion section, if you have a paragraph for a champion that hasn't got one, please share and you'll be credited.

Ok first things first, don't pay too much mind to the cheat sheet, it's not really right, I'll give you some more approximately correct values for your stats further into this.

This guide in essence is building Heimerdinger as an AP champion, whilst there are many survival elements in this guide, it's not optimized for your survival as if you do it right you should barely die and when you do it's because your ultimate and ghost are both on cooldown.

It is a challenge to play the opening of this guide out, simply because you're using Heimer who has a weak start as it is, and not making him stronger, so you are required to play smart, however the huge pay off comes at the end when your turrets do 300+ DPS each, your rockets do approximately 1000, and your grenade a similar value with it's nice stun for your turrets, you could single handedly thrash the turrets hard as it may look.

Stats -
AP value is 600-800 depending on how well you're doing
Mana will be close to 4000 sometimes
Move Speed is 381
CDR is 29%

well read on.

Well for Runes there isn't much to say.

Marks: Greater Mark of Insight for your magic penetration is quite nice, especially when you have the Abyssal Scepter, and they haven't stacked any Magic Resist, which is surprisingly likely if you're the only AP character which you'll see happen quite often in 3 v 3, the enemy will be taking the full force of your turrets, and generally on any mage, you use insight.

Seals: Mana Regen, if you're the carry which you should be or at least laning alone, you'll have to maximize the amount of time you can stay in the line for, which needs to be at least 4 minutes and 15 seconds, the cooldown on your teleport, Heimerdinger's turret cost starts high and does grow, and when you have to keep putting them down due to destruction or pushing forward mana depletes rather quickly, which is why it's advised you buy the sapphire crystal first, for two turrets worth of Mana.

Glyphs: This is open ended, you can put health regen, mana regen, magic pen or even AP runes here, anything that you personally feel you need, you're going to get more bang for your buck if you get magic pen as opposed to AP though.

Quintessences: One of Vigor and Two of Fortitude, this is an extra 50 HP which will generally help keep you alive, and because Heimer can happily fight away from the fray, health regen can afford you going in more often for rockets, grenade and turret placement.

The masteries are as is, there's not really anything different than a standard mage build, but I'll explain anyway.

Utility: The utility portion of this, increases your mana regen which after a few plays you'll see how important it is to have this quite high, the greed mastery doesn't yield much only 6 gold per minute, 60 per 10 minutes and around 200 gold the whole much, the only purposes this serves is when you're that little bit short of the next item, you'll be less short with this, and it can pay for any potions you want with this, pays for one mana and one health potion every 10 minutes.

the 6% cooldown on champion abilities is a little more than half a second when combined with sorcery and Heimer's ultimate yields a total of 29% cooldown reduction which is 3 seconds on rockets and grenade, and it makes turret placement lag 0.7 seconds more on that later, and you get this great stuff without having a cooldown item, great right.

Presence of the master, this is probably the greatest mastery you could ever spend as a carry or solo laner in general. This cool perk takes 45 seconds off the teleport cooldown, which allows you to have a more powerful presence in your (and others later on) being able to recall with teleport to go is good for when you barely survive that gank or are sitting on 2-3K gold, which could seriously crush the enemy carry. This also helps with your ghost not so much but it does, which is Heimer's best way out of a gank combined with upgrade.

Archaic Knowledge: 15% Magic Penetration, unless your opponent has any additional Magic Resist, this is useless once you have Sorcerer Shoes, as they'll have 0 Magic Resist against you anyway, and % Magic Pen is applied after flat and 15% of 0 is 0, however if your opponent is stacking Magic Resist, when you take void staff, you cut through over half their magic resist, which will screw them, as they've got more items focused into tanking and not damage output.

The extra 3% move speed puts his standard move speed at 309 which takes him off the baseline of the slowest champions and helps from him being completely chased down until you get shoes, the ghost mastery stacks further so you can get away from early game ganks when your enemy isn't going to stand there for your grenade stun and when you don't have two powerful turrets down fast and the 30% slow from Upgrade to help.

If you play in a premade where it's a tactic to take Baron Nashor down, take utility mastery instead of awareness, awareness isn't very important after level 3, when you can get a good out leveling going on, as a carry or solo laner which is what you should be doing awareness isn't particularly useful so it's flexible with your summoner spells.

Don't use good hands, you shouldn't be dying at the end of the game when death is a problem and when 10% makes a difference. If you're not using ghost however for whatever reason, the teleport mastery is still pretty useless so you'll get one mastery point in good hands, this only means 2 seconds though at the end of the game.

If you're looking to be a 2 v 2 laner, you'd probably being looking at 21/0/9 so you get the mana regen and increased damage output, the 45 seconds on teleport isn't as useful as you have a partner to cover you.

This is a BIG section, prepare yourselves, popcorn and a bucket is advised.

Catalyst The Protector: This should be your first item because as you level early game it keeps you in lane and helps with those mana pinches, when you can just hang around to level up and take the happy partial refill in your mana. Generally take the sapphire crystal as your opening item, you can take two health potions as well if you want, if you're doing a 3v3 however and you're a team who goes and likes to take first blood if not get two or all three of them, take the Ruby one, to optimize your survival and to make you battle effective.

Tear of the Goddess: You want this item early to fix your mana issues and build more mana for later when you get Archangel's Staff it's worth 30-40 AP to build this now as opposed to when you're making Archangel's Staff from scratch later on. Once you have this you won't be leaving your lane for mana issues, it's a pretty solid mana item, which builds to a solid AP item.

Mejai Soulstealer: You're looking at level 7 roughly by the time you get this, level 7 is when you have two turrets your ultimate and your rockets to do 100+, if you're against a melee opponent you can start getting kills, if you're against a ranged kite them a bit, and let them come off their turret if you're in a 1 v 1 lane, and then you'll be able to pick up kills, this is the only AP you have for a little while longer so if you can rack it up, you can be forceful sooner rather than later, once you've got 10 stacks, be more cautious until you have Deathcap, this will optimize your AP and make you significantly more powerful, also at 20 stacks, and level 11 you have rockets and grenade on a 6 second cooldown which is important.

Sorcerer Shoes: Up until you get these you've been dodging ganks with ghost, which isn't ideal to be dependent on, it's only so late because of early game items, if you're having trouble, build these after Catalyst. You'll get roughly a 10% damage increase on an enemy champion who hasn't got any additional Magic Resist. Move Speed should be in the 380s now, a regular speed, you'll have ghost and Upgrade for dodging Ganks by faster champions.

Rod of Ages: This may not give more AP than Archangels Staff despite their similar replace, but you want to make use of the growths, so this comes first, and it generally increases your survivability and thereby lane endurance. By the time you get Deathcap this will yield about 140 AP, mana to Archangel's Staff and the 30% increase from Deathcap still makes this good bang for your buck, you won't be leveling much by the time you get this so the Catalyst effect is redundant.

Archangel's Staff: A good AP item, and at this point it's extremely cost effective, it's cost is slightly higher than the Needlessly Large Rod which as long as you have 1000 Mana you'll be matching, and you should have 2-2.5K.

Rabadon's Deathcap: Bottom Line, once you've got this you're a god enough said, if you're at 1400 Gold hang around a bit kill the wraith spawn till you get 1600 it's better to get the needlessly large and then stay around until you can finish Deathcap, as opposed to get Blasting Wand, then go back again for Large Rod, and then again for Deathcap, your AP should be over 600 now, it'll be over 800 if you've been playing well and have a large amount of stacks on Souleater.

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The final item, there are a multitude of choices, in a general situation Zhonya's Hourglass is the most effective but I'll cover the possible choices.

Zhonya's Hourglass: Makes you a lot ******** harder to kill, think about how much damage your turrets can do in 2 seconds, and on top of that, your cooldowns might refresh for another go with your rockets, so not only can it save you in normal use but it has it's offensive, AD champions will also do less damage because of the armor.

Deathfire Grasp, you don't care about the cooldown reduction here (it will take a second off rockets and grenade though), you want the active, and with your AP and magic pen you can cut someone's HP in half, great for thrashing a high health tank for no effort.

Void Staff: Your counter to them countering your high AP with magic resist, this with Archaic Knowledge is going to go through half the magic resist.

Lich Bane: You can make use of your abilities how about 1K attack damage? cast an ability and then attack, rinse and repeat, it's also a good counter to high magic resist champions, who haven't stacked armor, champions that are just countering you, not being a tank in general. This is probably the best for Damage Output, but it's also almost twice the price of the Abyssal Scepter. If you want this get another of the above first, and then sell it when you can make the cash up, shouldn't take very long with the kills you should be getting.

Rylais Crystal Scepter: You should take this if you're feeling squishy, you don't need it for the effect or AP, you need it for the HP without compromising AP loss.

Abyssal Scepter: If you're enemy hasn't been stacking any MR and you have a couple of AP/magic damage champions, this can work greatly in ganks, making your team do more damage as opposed to just yourself, it also helps protect you from the enemy casters.

Final Note: if you want to stack AP to sheerly stack AP, A second Archangel Staff will exceed the effectiveness of a second Deathcap, more mana for the first, and more to stack on the second and the 30% increase from the first deathcap, can make for a 200+ AP jump with a second Archangel Staff.

I'll be highlighting skill level importances and certain level importances here.

Starting out: Take your turret is usually the best idea, even going for first blood the turret will give out more damage than your rockets, or you'll give out the damage while they destroy the turret, in a 5 on 5, run into the middle of the middle lane and sit it down.

Level 4: At this of these level you won't be able to level your turrets, if you're opponent is pushing his luck and you think you can get a cheap kill in, level your rockets to 2 instead and fire them when they come for you, your turrets will do a little as will your normal attack, your rockets should be able to get them, this is situational, it's normal to get the grenade, stun them, use rockets and get them to recall, you're not aiming to kill yet, only being an opportunist.

Turrets: First priority to level, level 5 is the most pivotal, simply having two turrets makes it significantly harder for your enemy to get off the turret, in a 1 v 1 lane. The longer they try to stay around, their magic resist will fall due to their level 2 effect, the level 4 effect makes it harder to destroy them, in the time frame for new parts, which at this point is what the enemy should be trying for, level 5, you'll devour minions waves extremely fast and get most of the kills, it's easy to get 300+ minion kills with Heimer because of his turrets.

Rockets: Your personal damage output really, scales reasonably well with AP, and is great for taking out the retreating enemy champion, especially kiters, most champions have to be in rocket range to attack you at all, as long as there aren't many enemy minions around you'll get them, either killing them later game, or making them recall early game.

Concussion Grenade: This is the hardest to use skill of Heimers it's also the least important, it is an AoE stun however so one level is good, it's a skill shot which has it's pros and cons, generally it's a Trynamere's worst nightmare, half their undying rage time is taken by this, and you won't miss with how close they are, you'll almost always get a melee with the stun, the blind makes Tryndamere's and most Yi's useless as they stack attack speed and attack damage. If they're going to try and run, throw it toward their turret, toward the end game it'll probably kill but your rockets should be back by now to finish the job anyway, stunning at range with this is incredibly difficult.

Upgrade: Only good when you're winning is a good summary to the active portion of this ability, it makes running harder and getting that great stun in easier, but they need to run first, however it does restore health to the turrets so early game it's useful to prevent your opponent pushing you in the 1 v 1 lane. 5 Rockets is a great way to get a multi kill in after the enemy team has just ganked your 4 team mates and now going to retreat to recall, you can just get them all, and that's plenty of stacks for Mejai too.

My most advised spells first off.

Teleport: Gives you a good field presence for most characters, but it lets a solo laner keep their lane in one piece a lot more effectively, and they can have comfortable recalls be it for mana or items or health where necessary, for the end game, getting you across the field to take a turret is very nice, and because you can teleport to your own turrets, can make it very easy to do.

Ghost: Your gank dodging mechanism, generally a better dodging tool than flash, and flash lacks offensive purpose with Heimer, shorter cooldown makes it more valuable.

Clarity: This spell works early game, however it's a waste of a summer spell at the end of the game, once you've built runes and masteries to compensate for mana it isn't really needed at the beginning anyway.

Exhaust: Great for keeping the melee champions useless against you, and in your turret range for longer, easier stun with this, it's one to bear in mind.

Rally: This has some use, it works on your turrets, you're already an effective pusher though so it's not needed and for killing a champion with it, exhaust is a lot more effective.

Heal: Only purpose it serves is to survive first blood in 3v3 whilst taking the Sapphire Crystal.

Cleanse: Not really needed.

Clairvoyance: Competent teammates and wards render this useless.

Ignite: main purpose for early kills, and halting their regen for when they try to run from you.

Smite: Don't bother, unless you have a phobia for tibbers.

Flash: Ghost does a better job for Heimer.

Revive: Just no.

Heimer is better suited to laning alone than with a partner, mainly because he's a great farmer and you'll probably starve your partner.

Summoner's Rift:

1 V 1 Lane: For all intensive purposes this is for the center lane, I'll add notes about doing a 1 V 1 in the top or bottom lane, so first things first, if you're doing this lane you are the carry, and you haven't got a lot of help, it's just you against their carry, your turret placement will be the deciding factor as to whether you push or are pushed, there are clever ways to do both.

To push, build your turrets in the middle of their minion wave, you might take a hit from the enemy champion but that'll be about it, the turret will then start to tear through the enemy minion wave, and suck up some hits for your minions to do more damage, the enemy champion will likely destroy it, until you get to level 5 you won't be able to keep them at their turret, when you have two turrets position them to the side from the turret near the ridge where the minions will walk as they come round the turret, your turrets will more or less kill one minion before they reach the turrets, even more when you reach level 9, keeping them against the turret, protects yours and whilst they'll still gain exp they won't gain as much gold where the turret kills your minions instead.

Note about AoE enemy champions: To outmaneuver an AoE champion place your turrets to the side of the minion waves, if you're pushing with two turrets, alternate sides, it'll take more effort to destroy them on the champion's part. When holding them to the turret, only place one in the above position, and place the second in line with the turret but further back, so the minions will go from the first turret to the second turret.

Now to be pushed, the reason for this is a tactical retreat whilst minimizing your experience loss, rather than just running off, fall back slowly, to do this, place your turret behind the melee minions and the enemy minions will slowly push you back, all the while you're still last hitting and they probably aren't as they don't want to be in your turret range, as you get onto your turret, place your turrets either side of the turret, you can put them slightly in front if you have a melee champion as the opponent, you'll still last hit, keep your turret safe whilst you recall and teleport back and so on. A good part to this is it's easier to force your opponent to recall, get back quicker and push them straight back onto their turret, reduce their wave to two near dead minions, then fire your rockets, hopefully with the enmey champion in range, if the enemy champion is then in range of the turret, you'll rocket them and the turret will target them, taking a good chunk from their health, and if they've been pushing their luck you'll probably kill them. If you can stun them first, you'll get more damage done and make the the main turret shot more likely.

Now to actually take their turret, it'll be a slow process to do it properly, but you let minions do most of the work, if you have two turret charges, smack one down to add in some more damage, kill their minions so they don't kill yours and you'll wittle it down, eventually you'll kill their champion or make them recall at the least, when they have no champion there put both turrets down and go hell for leather, if this happens twice you'll have it.

Preserving your turret, your first main turret is essentially the most important turret in the game, this front middle turret makes it harder for the enemy champions to push bottom or top lanes with multiple champions, so by keeping it standing your helping both the middle and top lanes as they're less likely to try anything for the time it takes, where they could lose a turret in the other lane, and in general the best way to win is to go clean through the middle and take the other two inhibitors from the inside, that's a lot harder if this turret is still standing.

The last section here, gank evasion, you're susceptible to ganks from both sides, being killed in a gank can cost you your turret, so there are a few steps to prevent being killed in a gank.

Competent Teammates: This is the easiest way, the most effective, and the cheapest, your teammates simply call SS on team chat and you fall back quickly, avoid the gank and all they get is their carry comes off their turret.

Wards: Sight wards, can cover your *** anyway, just put them in the bushes to the side, two in each works, and you'll know when someone comes to get you, this however stops working as effectively later on when you've taken the first turret or they come toward you after being recalled.

Escape: How do you escape from these damn ganks, you have 3 tools at your disposal, your grenade, your ultimate and ghost, you shouldn't need all three, and ghost should be the last one you use, because it has a longer cooldown, your 1.5 second stun from a grenade even on just one champion can get you out of there, if your turrets are down and they have to go past them hit your ultimate and then they should be slower, if at this point they've used flash or ghost or both, you'll need to hit your ghost button and get the hell out of there.

Just a final note to help with boosting your gold value, the wraith spawn are worth about 70 gold each, your grenade followed by rockets will be able to kill them, you'll miss out on 12 gold to start with unless you activate your ultimate which would be wasteful, once you get to level 14/15 you should be getting the full value, you can do this to your enemies wraith spawn if you have them against your turret, you can do this without leaving your lane, and it helps you get that deathcap all the more sooner.

This scenario will occur from time to time, and if it does, make someone else do the middle lane, you as Heimerdinger are the most effective 2 v 1 laner, hands down, provided you know what you're doing that is.

Okay doing this requires you to keep the enemy on your turret, this opens up opportunities later on for what should hopefully be your jungler to gank them, your turrets need to be positioned to keep last hitting the minions, bear in mind the range and AoE's of the opposing champions.

Keeping the enemy on your turret, over time minions and your turret will allow your minions to move forward, keep in the experience and JUST last hit minions, you'll end back up at your turret quickly, whilst still optimizing your gold and exp gain.

Forcing the enemy to recall: On principal your rockets and/or grenade will achieve this, as minions run low, you'll eventually where them out, you probably should buy the Meki Pendant first to do this, as your recalling should be rare, 2 enemy champions that close to your turret is a bad idea, so you ideally need to make them recall first or at least one, keeping them on the turret will cause the turret to get the odd shot when they slip up, early game that's a good chuck of their health.

Getting Kills: You're best opportunity to get kills is when you're at least level 9 and the enemy champions haven't became level 6, level 6 isn't always a problem depending on the ultimate's of the enemy champions, going in for the kill is generally done in the normal manner, stuns, rockets and turrets, if they try to fight and then run, hit your ultimate and get them with your rockets again, you can do this alone.

A note about this, the opportunity doesn't always present itself, and if you're confident for this rush Mejai Soulstealer, you're probably better sacrificing Catalyst the Protector, you're going to have leveled to fast by the time you get it, if you aren't going to do these kills or are too inexperienced to leave the turret for it, you might not want to bother with Mejai at all, and get Lich Bane at the end of your build, for some nice attack damage.

Leveling Up: You will drastically out level your two enemies, and toward the end function as a second carry, unlike the middle lane who should have taken a turret or two, you probably haven't this is okay, as your difficult task should provide your team with a late game win, because your jungler that you've been covering for will be stronger than the 2 v 2 laners of the enemy.

Keeping two enemies in your lane: Keeping both enemies laning in the earlier stages is important, you need to make them KNOW that one can't afford to leave the lane, and to do this is simple, kill the one left behind when the other leaves, this will compel the enemy champions to stay in the lane as opposed to try to gank mid, there's not a lot you can do about this before level 5 or so, which is the time they have to respond to your jungler after that it'll be too late, but you can do a 1 v 1 lane if they do this early.

Important Notes about 2 v 1 Laning
1. Make it happen in the top lane, this is because you'll have a harder time to defending and killing the dragon alone, as opposed to two champions in the bottom lane.
2. You MUST have teleport, you can't afford to leave your lane for more than 20 seconds, and even then you don't want to do that, unless you know and trust your jungler to do this.
3. Your second summoner spell MUST be able to help you escape, Ghost, Flash or Cleanse based on your preference, go for what suits you most, but I personally advise ghost, as it's more reliable and has a shorter cooldown, if you're playing a more cautious game Flash will serve you better as it can get you back in your turret range quicker if you're not going far.

Even though I HIGHLY advise you DON'T do this, I'll give you guidelines on how to make it out.

Bottom line is you are going to starve your partner to some degree, so to make the lane successful your teammate has to be individually competent so you can get champion kills in order to keep your partner going.

Starving your enemies: Heimer alone will have a problem starving his enemy(ies) but with a partner they can go behind the enemy minions and you stay toward the middle somewhat safely to guard your ally if they try to take him out, you get in with your stun rockets and adjust your turrets if necessary, although they shouldn't be far from your ally anyway being in the the middle of the minion wave.

With this tactic even though you'll be starving your partner, you'll be starving your enemies from most of their gold and even a good amount of experience if done right.

It's a good idea to go bottom given a choice, you're a good pusher so will be able to make room for your team to get dragon, and by placing turrets toward the river you can guard dragon, from the enemy bottom lane champions.

Note: Do NOT under any circumstance lane with Jax, Veigar or any other carry champion, they'll remain weak for too much of the game, you're better off with a balanced champion who items allow them to keep up with other champions but aren't particularly necessary to allowing them to do well.

I believe I've sufficiently covered this already within other parts, but for people looking specifically for effective farming tips with Heimer, this section is here.

Last Hitting: General mechanics of last hitting apply, your turrets will likely last hit plenty of the time, once you have rank 5 turrets their splash damage will mean minions get less kills than you last hit, positioning your turret in the middle of a minion wave, this is mainly to make them more effective because of your turret stockpile, avoid putting two turrets down for one wave, unless your trying to get champions.

Clearing a Wave: Later in the game when your pushing for more gold for Deathgap and your last item, to really help you kick ***, drop your rockets to take out the melee minions, and throw your grenade for the spellcasters, you'll destroy the whole minion wave reaping about 200 gold.

Jungling: You should only be in the jungling moving between lanes, but taking any wraith spawn, wolf spawn and the golems en route can increase your gold increase and decrease the enemy junglers intake, your grenade can attack all monsters of any minion camp and get the stun, then fire your rockets, this even in early stages of the game will kill the whole wolf camp, for the golems keep going, try not to put your turret down, for the wraith spawn only only one will be left so just finish it.

Avoid killing the buffs, other champions on your team will benefit more from having them, however firing your rockets toward an enemy jungler can keep your enemy from taking them, if you've been solo laning you'll likely to be able to kill the enemy jungler without much effort, this will need your turrets so they will target the buffs afterward so finish the job afterward, if you disrupt the enemy jungler enough they should join the lanes or team and without the buffs, bonus.

Dragon: With a partner you can kill the dragon quite early, your turrets and rockets will have a good output, you can do this at level 9 if you're having a good game and your Mejai has been snowballing, otherwise wait till 13, once you've got deathcap you should solo dragon regular, more for your team mates than yourself, but don't leave your team mates when they need you. If you're laning bottom, you have an obligation to take the dragon out as regularly as possible.

I'll be using this more to explain how each of Heimer's techniques works, their relative effectiveness and when the best time to use them is.

Turrets: Your most important skill by far, turret placement is key to your individual and team success, two turrets can make for a perfect ambush in the bushes near the inner turrets, bear in mind this will only likely work once on each champion but it gets you stacks, this can force another quick recall and push the turret.

In a team fight you're quite successful, your turrets will have a good damage output to aid the team in getting kills, assists and thereby stacks, if you have an AP heavy team, the level 2 turret perk, will lower their magic resist, meaning they'll quickly become more vulnerable to magic damage, including your other attacks.

Also in a team fight if your turrets get focused activate your ultimate to keep them around a bit longer, if you get focused which may happen, I suggest getting Zhonya's Hourglass and using it's active this will disrupt their attack, and allow your turrets to go for longer.

Try putting your turrets behind your allies to give them the best chance of doing the most damage, if their are more than 3 champions around, activate your ultimate and fire rockets to get all of them for what will later on be 1000 damage and probably a kill.

Rockets: The key to using rockets, is spatial awareness, knowing what you're going to hit, this takes a while to get used to, but they have a great damage output and it's no messing around selecting your target, great for multi kills and getting a lot of damage done in a team fight.

Grenade: Your grenade is a great stun, it's stun at the beginning is one of the strongest there is, and it's blind is great against AD/AS champions in teams fight, making them useless, landing the grenade is the hard part, in a team fight, throw it in from the back, if they run, it gives your champions a break and lets them do more damage. If you're being attacked by an AD/AS champion drop it on yourself, and bayum stunned and blinded, rockets away, put turrets down and you'll probably get them, this also makes Tryndamere useless for half his in undying rage, cool right?

Upgrade: The better parts to this, is it's passive that takes 2 seconds off your rockets and grenades, with masteries you're up to 3 seconds. When it comes to it, good damage in team fights with the extra 2 rockets, faster grenade, and restoring your turret health, and will stop them getting away as easily. Don't bother popping your ultimate for the sake of moving your grenade faster, think of it as a bonus when using it for the other things.

Offtank Heimer is your main alternative to playing Heimer, you shouldn't aim to play as an offtank, but play one if required, the build doesn't change much and where it does is towards the end for the most part, there are better ways to offtank as Heimer but that's if you go in with your head screwed on with intent to offtank.

Reasons to Offtank:
1. Too many allied squishy champions, if you have champions such as Ashe, Annie, Akali etc and you only have one CC/Tank or no tank at all, you should offtank, to allow your ranged and burst champions more time to kick the living hell out of the enemy champions.
2. You're feeling too squishy yourself, if you keep getting killed because you're struggling to escape or just generally be burst killed repeatedly, start offtanking to get the enemy burst champions.

This offtank section is built in mind with preserving your high AP whilst making you harder to kill, however your damage will be smaller.

Offtank Build: Start the same more or less, maybe rearrange the items, but you will still want the items up to sorcerer shoes, your last items should be Zhonya's Hourglass This will disrupt the enemy attack when you're being focused, hopefully shifting it to the other tank, or even just allowing more time for your bursts and AP nukers to kill/stun your opponents and get the job done, it'll also has 100 AP and some armor which means AD heroes do less damage anyway and it doesn't really cost much of your own. If you have AP bursters/nukers take Abyssal scepter for magic resist and lowering your opponents MR. If your team is split on AD/AP attackers, go for Rylais Crystal Scepter, this'll help you keep the enemies pinned down, make you harder to kill and still preserve your AP.

If you're either having a hard time or generally want to go more offtank take all three of the items.

There's nothing wrong with Offtank but use it when you have too I advise against planning to use offtank Heimer as opposed to an AP/Survival Heimer.

So you wanna be a burst Heimer? don't.

Heimer isn't born to burst, he's meant to hang around for the whole team fight dealing high amounts of damage over a large period of time, ie. the whole team fight, but for those of you looking to get bursty, start with archangel's staff, sorcerer shoes, Deathcap, Lich Bane, then two more archangel staffs, if you're on a 3v3 you won't have the money for this, so build a Mejai either before or after Deathcap, this relies on skill and seeing as heimer is slow, take flash and ghost to escape, to preserve your stacks, and generally not end up dying on a regular basis.

Burst Sequence: Rockets --> Attack (Proc Lich Bane) --> Grenade --> Attack (Proc Lich Bane) --> Both turrets down

rinse and repeat, get the hell out of there if your health is going down fast.

This section will be pretty large, but it's good for when you're against those pesky champions you don't know how to deal with, and it can help with knowing what champions you can effectively destroy which many champions can not, the list will be alphabetical and updated regularly.

Akali:

Alistar:

Amumu:

Anivia:

Annie: General Annie combat applies, kill her in between her use of tibbers, if tibbers drops you with stun (like a good Annie should) you are likely dead. Going Offtank with Mercury Treads can work OR just do it normally and activate Zhonya's Hourglass every time she uses Tibbers, Hourglass CD is shorter than Tibbers CD most of the time, unless the Annie has stacked CD and then it'll be less, but she's less threatening if she's stacked CD.

Ashe:

Blitzcrank: His ultimate can easily thrash both your turrets and you, so spread yourself around, and when his ultimate isn't happening hid behind a turret or minion to avoid rocket grab, your turret will not be affected in any way by rocket grab.

Caitlyn:

Cassiopeia:

Cho'Gath:

Corki:

Dr. Mundo:

Evelynn: You could just Oracle's Elixir, but then a smart Eve won't come near you and hence you won't be able to kill her, if you want to kill eve, just progress slowly when she's MIA always standing between two of your turrets, when she stuns you and goes to kill you'll burn most of her health before the stun ends, you shouldn't be dead, then drop your grenade on yourself she'll be stunned for 1.5s your turrets are already positioned fire rockets and kick her ***. If the enemy team has an eve lane 2v2 against her, if she's in a different lane she could be fed too fast for you to do anything about it.

Ezreal:

Fiddlesticks:

Galio:

Gangplank:

Garen:

Gragas:

Heimerdinger: Fighting yourself, truth be told, this match up isn't going anywhere, you know each other's moves and know how you put damage out and thereby your opponents but the same goes for him, keep his turrets down is the bottom line, even if you have to use rockets, the easy way out here is to defend, he won't be able to fight you on the turret, you'll need to then call a gank in to get him from behind, once he's out of the picture move up as far forward as you can, put two turrets down and use that as a standing point, you'll need to adapt a build to counter him, Lich Bane works well cause he probably won't be stacking much armor, and if you have 700+ AP you'll go through him quickly. Fortunately Heimerdinger is an acquired taste of a champion and hopefully when he is in the same match, you won't lane against him.

Irelia:

Janna:

Jarvan IV: This champion is surprisingly easy for Heimerdinger to beat, when he comes for you, just put your turrets, wait for him to pop his ulti, throw your grenade more or less on your self, and you've stunned him for 1.5 seconds and blinded for a further 1.5 seconds, this makes his ultimate useless, given you won't be able to use your grenade in 0.5 seconds, make sure you've used rockets preferably before he starts this, once it's over, rockets and ultimate will probably kill him, depending how far into the game you are. If you have to run use your ultimate and get out of there.

Jax:

Karma:

Karthus:

Kassadin:

Katarina:

Kayle:

Kennen:

Kog'Maw:

LeBlanc:

Lux:

Malphite:

Malzahar:

Maokai:

Master Yi:

Miss Fortune:

Mordekaiser:

Morgana:

Nasus:

Nidalee:

Nunu:

Olaf:

Pantheon:

Poppy:

Rammus:

Renekton:

Ryze:

Shaco:

Shen:

Singed:

Sion:

Sivir:

Sona:

Soraka:

Swain:

Taric:

Teemo:

Tristana:

Trundle:

Tryndamere: Tryndamere's 99% of the time are going to die against or run away, with undying rage. Many will try killing you, for those that do, your grenade will make them useless for half of it, with blind, considering most Tryndamere's are AD/AS, it's useless if they're blinded, for the other 3 seconds you should be fine, and get them with your turrets the moment it ends, if you're struggling to live, hit your Zhonya's Hourglass active up, and it'll be over by the time it finishes, your turrets finishing the job, if he runs early, try giving chase, and firing your rockets provided he hasn't reached a minion wave, Tryndamere is not something to be worried about.

Twisted Fate:

Twitch:

Udyr:

Urgot:

Veigar: Veigar's ultimate is a definite nightmare to you, so you want to head in the offtank direction, to make you more durable and so his ultimate will do a little less to you, seeing as it scales off it's targets AP too, whilst this makes you better for fighting Veigar you still wouldn't want to hunt him down late game, if you see his ultimate on route to you, hit Zhonya and you'll be okay, if you're struggling because it's a good Veigar who can place his stun well, take Mercury Treads instead of Sorcerer Shoes, this will make the stun shorter by over a half a second and make his attacks do slightly less damage due to the magic resist, Abbysal Scepter is a good compliment on the offtank build for him, the AP provided will not increase your damage taken, because of the magic resist.

Vladmir:

Warwick:

Xin Zhao:

Zilean: