AP master yi, the Assasin, Soak-offtank, and Anti-carry
Many people think AP yi is a troll build, and not really viable but i think its decent, and he offers a bit more utility and more team fight wide burst without having to be directly ontop of the squishies. He can tank towers at full hp better then most all champs which is excellent for helping your team mates dive or pushing. AP master yi also has one of the easiest and best laning phases with a 2000+ range nuke that can harass from a completely safe range, in addition to extreme healing with meditate.
Passive - Double strike:This isn't very useful for ap yi, might help last hitting, Sadly this cannot be used to double proc sheen or lichbane, That would be awesome though.
Q - Alpha Strike:This is your main skill, this is almost all of your damage, right into a nuke, This launches you at the target and jumps between all targets in range doing damage too them, with practice you can bounce this off of creeps for insane range harass. With a 1:1 AP ratio it nukes hard, You become invulnerable when using this for a short while which can be used with perfect timing to dodge aoe's, skill shots, and things like karthus ult.
W - Meditate:This is your **** you button, It gives you a ridiculous armor and heals you at the same time, but there is a catch, you have to stand still and channel which means it can be interupted which makes you very vulnerable and people can and will try to ruin your meditation because they get very angry when they auto attack you and you stand still and out heal all their damage, Still, this will let you win almost any 1v1 if its not interupted, and can really frustrate early game fights and ganks as you will generally always out heal their damage, This is also amazing for tower tanking late game as well as tower dives, Tower diving can be extremely easy for AP yi, You can dive things and come out alive like no other champ can, You can begin meditating for the armor bonus and healing right before a tower shot hits you, then move out of range before it even shoots another shot for a complete freebie, and late game you can stand under the tower for the full duration of meditate and maintain full health. This is also great for baiting and low levels, once you get a feel for how much damage you can take and get out of you can bait extremely well at low hp and then have your allies come in for a gank or if they tower dive just start meditating.
E - Wuju Style:This is pretty useless, but its still damage, you should level this last, it can make a different with highlander on though, so in late game team fights don't forget to activate it, or even use it to charge or .
R - Highlander:This pretty decent, this lets you refresh your alpha strike for chain alpha strike snowball goodness, and it gives movement speed which is always great, and not makes you immune to slows, can even help you get out of sticky situations.
Most AD yi's use ghost ignite, or ghost cleanse which is pretty solid for yi, but with AP yi you really don't need ghost for auto attacking while running, Flash ignite is much better for increasing the range on your 2000 range nuke even further and escaping ganks and walls and such, as well as further improving the laning phase and kill possibilities with his nuke + ignite.
Laning phase!This should be one of your favorite parts of AP yi, AP yi is very easy to lane with and can shut down solo's (with some skill) or duo lanes with easy.
Begin laning by using alpha strike as much as possible to safely harass by bouncing off of low caster minions to last hit them while harrasing, you should have no mana problems and if you do get harrased down below half don't be afraid to meditate, It takes some practice but after a while you will "feel" how much damage you will be able to do and know when you can flash in alpha + ignite for a kill, Most people will not expect it, If you're not able to get any kills don't worry and keep up that easy farm with alpha strike, although be careful to not push your lane by spamming it too much.
Once you get you can continue laning or try to get some ganks out.
Team FightsSo, you're now at a stage where you're about to team fight with AP yi, Just hang around in the back waiting for the initiation, Find the first focus target and Highlander() then + + if you have to, Hopefully this should be a kill or two for your team, and here's the fun part, roughly every kill or assist with his ult up resets the alpha strike cooldown, So you can do it again, roughly every kill will double your burst and compound it, If two people go down it shouldn't be hard to finish winning the team fight, If you get focused wait until they blow stuns, then meditate , once you have rabadons it only takes around 1-3 seconds to gain full health before you can start pounding the team again.
Counters/WeaknessesChampions that will poop on your parade are generally anyone who saves ignite for your meditate, and champions like gangplank, katarina, MF, and Tristana who can reduce your healing by 50% with out ignite.
Champions with high CC like Cho'Gath, Blitzcrank, Janna and Alistar can really ruin your day as well.
Final CommentsSo there is my AP yi guide, Hopefully it will help some of you guys try out this fun and neat build, Don't be afraid to comment!