Many people won't agree with the heal spell, but I have been using it for a while and it is pretty good, your core here is the Mejai, so you need stacks, and if you miss your shield what can you do? Heal xD

About the ignite, because of the Litch Bane you have to get close sometimes, that time you can use it, it is a nice dmg boost, also heal reduction until your ult hits the target.

Your first goal is to get GP5, and farm a lot of minions, but don't make it too long to get an Eleisa's if you are planning it, get it sometime around level 10, so it will break pretty fast.

About Kage's L. P, you will probably sell it to get the needlessly large rod or the blasting wand, but only do it when you have 4 completed items, and only 2 left, one what is the deathcap, otherwise just let it get you golt.

Mejai's: As I said, your core, get it after you get some spell pen and the GP5.

Bonus: Spirit of the Spectral Wraith is a pretty good midgame item, if you wish, you can buy it, but later sell it for a WotA.

(I added this chapter after everything else)

Pros:
Your Steel Legion Skin rules.
You laught nicely.
You ks like a boss, no chance for your team.
You also burst nicely, so your team have to shut up and enjoy the show.
If you poke, aim good, they won't catch you alive.
Nice neutral minion steam potention.

Cons:
You didn't bring the Spellthief skin.
If you listen to your laught too much, your premade friend leaves.
You are squishy.
If there is a Veigar, you can only rely on your LONG RANGE KILL COMBO (see below), but you can pull it off (the "pull it off" part is a Pros)
With this build Poros won't like you.
You are still squishy.
If you fail a steal you are ******.
And they kill you because you are squishy.
And the last thing: if you are on the front line, dont even try to run, throw everythig in and pray, you are just too squishy.

Early you should be pretty defensive, keeping your distance (AP/lvl wont help too much that time), but because of this don't be afraid to get FB with/without/on the jungler, that's why you have heal and ignite, a bit early advantage when used correctly.

While you cs on your lane don't forget to gank, you will depend on your teamates lategame, and they won't tank for you if you let them die to the enemy mid while you farm, go down, help, get the advantege on other lanes.

After you get Mejai your role changes from cs machine to ks machine, either way, get things your teamates want as well and you will be fine.
BUT because of your lack of defense you have to ks* smoothly (*or call it "help"), you want stacks, and even if they get 4 of your team, it is considered good if you leave with a triple kill, you will deal much more dmg next time, and earlygame it won't affect your turrets much.

After your Mejai's has a lot of stacks and because of the gold advantage you have a nice Litch Bane you can try getting free kills by comboing the enemy down, or going in with a teamate and get the kill with a LB proc.

And finally, when you have the Deathcap, you should have that much AP that it hurst, so aim for their support/adc, and kill them with a long range combo, this way you don't get into the fight, 20 sec later you will have your ult back, and they will lack one man.

Long range poke: Q-E, simple enought, on lane follow it with a basic attack, if you are close, do one between the spells as well.

Long range kill: Q-E position-R-E activate, you don't want to get into basic attachk range this time, if the enemy WAS in ba range before, you ignite him there, but most likely you wont use the illumination more than once.

Close combo when attacking a fleeing enemy: Q-E-basic attak-E-basic attack-(R if needed), ignite early if the enemy can heal, ult if the enemy has tons of hp.

Close combo when running for your life but you feel like poking:Q-basic att-E(both parts)-basic att-W-basic att-repeat, you can ult if the enemy is pinned down by the snare and he would die, otherwise keep locking him down, shielding, nd poking with LB procs.

If you cant afford to poke: Q-E(first part, in between you and the enemy)-RUN, well, yeah, snare them, E them for the slow, and also shied their long range attacks, and ofc let your teamates get the dmg by running in front of them.

You will probably be able to combo there, but that takes time, and in a teamfight you wanna be fast, so do the LONG RANGE KILL COMBO (I should have given some epic name to this) on their team, aiming at the most OR the APC OR the ADC, depending on their death potential, you want the less enemies possible there.
Because of your AUG you will keep your mana even thought you will spam you spells (for LB procs) afterwards this, won't you?
Your goal is not a pentakill, so dont go for someone in the teamfight who is being killed in one shot by your ADC, the KS times are over, now you will be a spell machine, who shields the bruisers and protects the adc with snares.

PS: Don't offend your teamates before a teamfight, they will let you die, offend the enemies, they will try to focus you, therefore rushing throught your frontline, getting beaten while killing you.
-I don't SAY you have to act cocky with the enemy team, but I suggest it ;) make them focus the backline, who is not even on their screen.

When their Nexus is about to blow up, use you LONG RANGE KILL COMBO on the one in your team you kept talking to in the team chat, or stolen the most from you, that way you will look epic, or use the same combo on the enemy fountain IF their support or adc is about to respawn (start the combo when they have one sec left to apprear), and kill them, if you fail, the game is over, if you succeed the game is over BUT you will look even cooler. DON'T ult the Nexus, its so Season 2.

At the end, if you win, write in 'gg', if you lose, dont write anything, just look offended.
Losing is bad, so you should NOT do it.

The map is set on SR, but bsically good everywhere.
I also feel an urge to add this guide as "assassin"...

Well, the start of the guide was to make me remember everything, but I wrote a little much, they aren't too strict, but true, it works for me, you should give it a try.
Also, if I am in the mood I write a little more on the beginning.

(IMPORTANT: THE ORDER OF THE SUMMONER SPELLS ARE HEAL-IGNITE not IGNITE-HEAL, its better this way, trust me.)

Ask if something is not clear, and I will answer it this year.

PPS: I will SOON™ add pictures, or some editing, so the build won't look like a homework.