Hello! First and foremost, with this build you are not going to have a lot of health, but you are going to have a lot of effective health. So keep that in mind as you read through it. This is an extremely AP heavy build, as you can tell by the item list above. This build is going to rely on you actively trying to stay alive due to the lack of health in addition to trying to keep your carries alive (as any tank should be). However, with all the AP you're going to have more of a presence in team fights with this build instead of just being an ult bot. Also, this is the first guide I've ever done so I am completely open to suggestions and criticism :). Now, let's get down to it!



Pros
You don't need a skin to be awesome with him!
Strong harass and zoning power against most champions while laning
Strong utility
Gains AP from MR
Game changing ultimate

Cons
Very squishy early game
OOM easily while laning even with Chalice
Ult can be easily interrupted and countered if you're not careful

Runic Skin (Passive) - Galio converts 50% of his total Magic Resistance into Ability Power.
I don't really think there is anything to explain here.


Resolute Smite - Galio fires a concussive blast from his eyes, dealing 80/135/190/245/300(0.7 AP) magic damage near the impact location and reducing enemy movement speed by 24/28/32/36/40% for 2.5 seconds.
This is your main spell on Galio. It gives you harass and zoning power, significantly slows enemies, and helps you pick off and catch up to those pesky runners. By about level 3 of this spell, you'll be hitting squishies for roughly 20-25% of their health. You can see why this should be leveled up first.


Bulwark - Galio shields a friendly champion for 4 seconds, increasing their Armor and Magic Resistance by 30/45/60/75/90. Each time that unit suffers damage, Galio is healed for 25/40/55/70/85(0.3 AP). (-20% healing for each consecutive hit)
Galio's "oh ****" button. Being chased by that spinning Garen? Pop shield and run away unscathed. Got any kind of dot on you? Put this up and come out with more health than you started with. An overall fantastic spell. Once you get into team fights, you should focus on keeping this on your carry. It will keep both of you alive.


Righteous Gust - Galio unleashes a gust of concussive wind that deals 60/105/150/195/240(0.5 AP) magic damage to all enemies in its path. A directional draft remains for 5 seconds, increasing the speed of allies by 20/24/32/36%
Definitely has the most utility out of all of Galio's spells. If you need to save yourself or a team mate, cast this in the direction they're running and then Q the enemy to make sure you get away. If you're trying to catch up to an enemy, cast this towards them then Q to make sure they AREN'T getting away. Just scored an ace and need to get your team to that turret or inhibitor quickly? Throw this down. It has so many uses. I only take one point in this at 4 to make avoiding ganks easier.


Idol of Durand - Galio channels for 2 second, taunting nearby foes and blocking 50% of incoming damage. After channeling, Galio deals 220/330/440(0.6 AP) magic damage to surrounding enemies, plus an additional 5% damage for each attack suffered while channeling. Max 40% bonus damage.
This is the spell that makes Galio an asset to nearly every team and a terror to the enemy. It is a mass 550 range 2 second taunt that is also an AoE nuke. Sounds pretty awesome. The best time to use this is right in the middle of a team fight (also a half-decent initiate). If you manage to catch 4 or 5 members of the enemy team, you'll definitely get the 40% bonus damage. However, if you can only get a few then I recommend trying to get a couple creeps along with it. Obviously don't go out of your way to do that, but it's nice when it happens.

Tips and Tricks for Idol of Durand

YOUR TAUNT IS AFFECTED BY TENACITY! Remember this. Always always always always. If you need to, move or hit R again to cancel it and deal damage before people have the chance to run away or flash out.

Any kind of spell or item that removes CC or debuffs will also remove your taunt. Pay attention to who is on the enemy team and if they have QSS/Cleanse.

If you're low on health or no other team mates are in danger of dying, cast W on yourself before using this. It may seem redundant with the 50% damage reduction, but do it. (Hint: it gives you MRes)

If you're laning against someone like Trynd who loves to turret dive, bait them then pop your ult underneath your turret.

Be wary of champs with on-hit CC effects like Blitz (knock up), Garen (silence) and Xin (knock up). Try to avoid using your ult when you see that they have that ability activated.

Only when you can get a pentakill are you allowed to use your ult to KS. Don't be a greedy gargoyle.

Know the range of your flash+ult!

The taunt range is slightly larger than the damage range, and both are slightly larger than what the graphic indicates. Make sure you are familiar with these ranges.

If you have a Nunu on your team, synchronize your ults for a guaranteed ace or penta.

Required

You know, you love it, you better use it. Flash is mandatory on Galio for the same reason it is on Annie: ULT POSITIONING! Also a great escape tool if your kit doesn't cut it. Don't be caught without it.

Other Options

This is what I use while playing Galio. It is simply a phenomenal spell. It allows you to shop, heal, get mana, and get back to your lane in less than 10 seconds. Has numerous other applications in mid/late game.

I would take this if I were to be in a solo lane as Galio. He is a surprisingly good solo and this helps him out tremendously.

Some people love this on Galio and I can see why. His laning phase is very mana intensive. However, I've never found the need for it because of Chalice and my conservative use of E.

Runes

MPen Marks x 9
Should be pretty self-explanatory. Makes you more effective in a lane by breaking through base MRes.

Flat Armor Seals x 9
Makes those annoying AD champs less annoying to lane against.

MRes/lvl x 9
The reason I chose per level instead of flat MRes is because 1) Galio naturally has higher MRes and 2) Your starting items give you a bunch. Also, these will you give you more AP late game.

Flat Health Quints x 3
Galio is unfortunately very squishy early game for a tank. These help tremendously. If you find you can get by without them, invest in flat AP quints.

Masteries

I prefer to go the usual 9/0/21 caster set up. Grabbing points in the Flash and TP masteries. Not much else to explain here. It gives you MPen, a slight AP boost, and 21 points in Utility which is always nice.

You could also run 0/9/21 I suppose if you think you need it, but I would stay away from 0/21/9. Bad idea.

Starting Items and Early Game

+x 2
There are numerous ways to open on Galio, but this is what I have found to work best. It let's you use Q more and stay in a lane longer. Nothing to dislike about that. Other options include Doran's Ring, Null-Magic Mantle, and of course boots + pots.

I try to stay in a lane until I have at least 975 gold then B so I can finish up Chalice, grab a Null-Magic Mantle and.... A WARD. Wards are good. Buy them. Use them. Keep in mind that with the Chalice and Mantle you are getting a decent amount of AP as well.

After that initial buy, you'll have enough sustain to stay in your lane until you decide to leave. Once you do, use your Null-Magic Mantle to finish up Mercs, grab a Chain Vest, and grab a Catalyst if you have enough gold.

Mid Game/Core Items

All right, now we're getting to the good stuff! At this point you should have your 3 core items complete or very close to it.
"Whaaaaaaaat?! Rod of Ages? But I thought I was going to build that Catalyst in to a Banshee's!" Hold on a second and let me explain. I used to do the exact same thing, but I found that Banshee's is a pretty situational item for Galio. I'll go in to more detail later. Rod of Ages, on the other hand, gives 3 stats that you really need as Galio. His health is naturally very low for a tank, so this gives him a nice boost in that department. His spells are expensive and he has a small mana pool, so the mana is good. Finally, there's nothing wrong with more AP.

You're also probably wondering what the Chain Vest is for. During mid-game, Galio is very susceptible to AD champions, but not so much that it warrants rushing a Thornmail or doing something else drastic. I've found that grabbing that early Chain Vest will hold you over until late game when you can build it in to something more useful.

At this point your build may very depending on how much your team mates fed and how tanky you think you need to be. In an ideal situation, you'll want to immediately go for Rabadon's. A good rule of thumb is to have it finished by the time RoA is fully stacked since Galio should be farming like a boss by now.

Late Game Items/Finishing Your Build

After Rabadon's start building your Abyssal. I would recommend grabbing the Negatron's first since your MRes will be a little low at this point. You should get this regardless of how the rest of your build varies as it is one of the best items for Galio. It gives you a bunch of AP via the straight AP and the MRes along with an aura that makes your ult hit even harder.

After Abyssal, you get to do something with the Chain Vest. I like to go for a Zhonya's since I'm an AP ***** (and it's active is nice to use after your ult), but you have many other options. Feeling too vulnerable? Build it in to a Guardian Angel. Are those AD carries fat and completely shredding you? Build it in to a Shroud and eventually Frozen Heart(The CDR is also very nice on Galio. You should consider getting this regardless). What you do with it is completely up to you.

After you do something with Chain Vest, what you do is up to you!
"But wait! That's all 6 item slots!"
At this point in the game, Chalice is pretty obsolete so you can sell it and pick up whatever you think you need. I would personally go for a Shroud if you don't already have Frozen Heart from the Chain Vest. Keep in mind that you'll only get this option if the game is going on for 50+ minutes as the build I use is fairly expensive.

Situational Items

Note: I would replace either your final item/Chain Vest or Rabadon's with one of these if you need it.

I told you I would get back to it. Let me first say that if you decide you need this, decide early and get this instead of RoA. The only situation in which you should go for Banshee's is when you find your ult getting interrupted frequently.

Always a good choice if you're getting shredded by an AD carry.

Same deal as Thornmail, except works against AP. Works well with Warmog's.

As I've said, Galio has very low health. If you just feel more secure with a larger health pool, pick this up. I can't say that I recommend it though. Works well with FoN.

Never a bad idea on Galio. It gives you more health and adds a slow to your E and ult. If you like having a lot of health I would definitely recommend this over a Warmog's.

Laning(Duo)

Levels 1-6 are going to be rough in terms of last hitting since your melee attacks merely tickle. During this initial part of laning I usually focus on zoning my opponents and letting my partner get some good farm in. It's pretty easy for your lane to get a first blood since you'll be keep your opponents fairly low on health. Once you hit 6, let your opponents push the lane a bit and try to score a double and get some damage on their turret. It should be easy if you've been constantly harassing and your partner can put out decent damage. Once you hit 7, it's your turn to farm. Q > E caster creeps to one shot them then last hit the melee ones with autos. You'll be putting out some pretty good damage at this point unless your opponents are building tanky, so keep bullying them when you get the chance.

Laning(Solo)

As I mentioned earlier, Galio is a pretty strong solo. The only champions who should be able to bully you in a lane are casters who can out range you and have good burst or tanky DPS with a heal since they will just shrug off your Q. At level 6 you should try to burst them down with a Q > E > Flash > Ult > Q > Ignite combo. If you don't get the kill you'll force them to B, which is just as good in most situations.

During laning don't be afraid to roam a bit and help gank over-extended lanes with your ult.

Mid Game and Late Game

DON'T FORGET TO KEEP FARMING! If there are no fights breaking out and your team isn't pushing a lane, you should be in the jungle or in a lane clearing creep waves. Galio is an amazing farmer and should take advantage of that.

Team fights should be starting to break out, so get in there and do what you do best: use dat ult. Don't just blindly dive in to the middle of a fight, use it, and expect to win. Choose your battles. If your team is severely losing a fight, help them run away and recover instead of ulting. If your team is behind a little bit, use your ult to give them an edge and win the fight. If you don't have a better initiate on your team, Idol of Durand can be used instead. If you're doing this, make sure your team knows it and is right behind you because the enemy will quickly kill you and run away if they aren't. Your ult is also very useful while trying to push a turret if there are a few enemies guarding it. Don't be afraid to pop it under an enemy turret in order to kill them and take it down. Once again, make sure your team is with you and is already committed to this as well because if they aren't you are just suiciding under their turret.

Much of Galio's mid/late game revolves around his ult, but keep in mind that with an AP build your Q and E become fairly powerful nukes with great utility as well. Towards end game if you Q > E the enemy team soon as your ult ends and they're grouped up, that's an instant 400+ damage on a few more enemies, an AoE slow, and a speed boost for your team in addition to your ult. Don't always Q > E after an ult though. It is also a good idea to use them to save your carries along with W since they will be getting focused down. You're going to have to judge the situation and figure out what to do during the 2 seconds that your ult is channeling.

Thank you very much for reading my guide! I find Galio to be a lot of fun and I hope you will too with this build! As I said earlier, I am completely open to suggestions and criticism. Please leave any feedback you have in the comments section :)