I've been playing AP yi for more than a year and I noticed he can't always do what you'd expect from AP carry. If you don't get your resets you're pretty much doomed and to get it you must wait until YOUR TEAM gets someone low enough for you to alpha. This high risk high reward playstyle of AP yi makes him bad in solo que as you can't always expect your team to deal enough damage for you to go in (especially S3 where your DFG + alpha + lbane combo is not enough to kill a squishy anymore)

So, when to pick him?

The more CC your team has the better AD yi is. Champs like Jarvan IV, Nautilus, Leona, Amumu and such are your best friends (great initiation).

Remember, don't pick yi when your team is squishy or has no initiation as you will instantly die no matter how good your positioning is. If the enemy team has good initiation and yours lacks it then you won't have much fun and you will only be there for cleaning up and bdooring.


Pros:
-Amazing harass
-Great sustain
-Great roamer
-Ungankable after he gets lvl 6
-Crazy damage obviously
-Snowballs like a motherfu**er
-Yoummu's Ghostblade alone gives him amazing damage


Cons:
-Team dependant. That's pretty much it. Not gonna list obvious stuff like squishy etc. More often that not you can tell whether you're gonna have fun or not in the champ select.

With ap masteries and Doran's Ring you do just as much damage with your alpha as AP yi. This also gives you nice sustain from meditate although recent patch reduced it's AP ratio so it's not as good as it used to be.


Early game is tricky as your main harass ability pushes lane like crazy. If you're up against a squishy AP champ you can afford harassing him with alpha + wuju double strike. It should burn half of anyone's HP early on. This way even if the gank comes he propably won't be able to help the jungler. You can also play defensively and just farm until you get level 6. That's when you become ungankable and can do whatever you want.

Keep in mind that yi is a great roamer so dont hesitate to help other lanes. Simply push mid and go bot or top.

Master yi has a global taunt much like Teemo therefore bulding him as a glass cannon simply won't work.

Infinity Edge and Youmuu's Ghostblade are propably 2 most efficient damage items for Yi that's why I picked these as his only damage items. Honestly no matter which damage item you build next it won't improve your damage by much therefore it's much more beneficial to build defense items like:

Randuin's Omen - Armor + HP needs no explanation. If enemy team is AD heavy and you're dying from auto attacks and no aoe abilities this is the best option.

Warmog's Armor - I dont buy this one very often. I find FroMal much more efficient but if enemy team is AoE heavy I might get it so I don't die from random damage in 1 second.

Frozen Mallet - If you're ahead you can get this instead of warmogs. Slow + extra damage will make killing squishes much easier.

Guardian Angel - This one should only be bought after you already have one of these 3 items above. It has some nice defence stats but the main reason for picking it is to discourage the enemy team from focusing you. This works perfectly in solo que where teams lack coordination. With GA and say randuins you're not dying unless whole enemy team focuses you down (which should never happen unless you're really bad at positioning :P)

Mercurial Scimitar - Great against Sion, Veigar and such. Always get it when enemy team stuns you and focuses you down instantly.

Last Whisper - If you see enemy team is full of tanky champions you can take AD runes for early advantage as you will propably need LW anyway.

Blade of the Ruined King - Another good pick against tanky teams. Pick it against HP heavy teams.

Phantom Dancer - Squishy enemy team and you have no problems with being focused? Extra crit chance might come in handy.

Atma's Impaler - Want a phantom dancer with armor? Pick atma. Even after the nerf it's crazy good. Obviously only take it when you already have some HP.



Spells:

Flash is a must have. Ignite is also very important but if you're up against a tough team it could be better to pick:

Cleanse - You won't need to buy Mercurial Scimitar. Only take it against teams with heavy CC

Teleport - This is crazy good for backdooring and ganking.


I'd still take ignite 90% of the time as it's really difficult to get a kill early on without it.



Skill order:

You don't have to stick to the skill order above. You can put more points in meditate or wuju style and less in alpha if you want. It all depends on how you're doing in lane. If you're losing you should propably leave wuju at rank 1 (as it only gives 5 more) and damage every time you upgrade it)and spend points on medi and alpha equally.