This guid will be very short and to the point. I play this build all the time I believe i have it down So i feel no need to explain myself any more than i do here.

In fights try to time your first alpha to get a kill while you're ult is on,
Kill with ult= Insta 2nd alpha
Assist with ult= 1/2 cool down

This is very important for AP YI, You must get Kills if not hopefully Assists
Ult->Alpha->Lich proc=Dead On any squishy Caster or Squish AD carry.

When attacking Tanks
DeathFire->Ult->Alpha->Lich

As for AP YI i dont believe there is any better burst caster out there if he is played right.

I like to solo mid as ap yi personally, Harass as much as possible steal Wraiths when you have kages and on. Deathfire->Alpha-> ignite + an auto attack or 2 should equal easy kills earlyish-mid game. I some times pick up Sheen before Rab's but not unless im dominating (it helps ganking the higher health top and bottom lanes.

In Team fights Dont be the initiator Wait till there is some dmg down or a squishy is in range for you're full combo if you feel Deathfire is needed by all means DO IT. DONT FORGET TO HAVE YOUR ULT UP BEFORE YOU GET A KILL!!!!

You can jungle Early Mid game (thats NOT early game its early middle)

Stealing Wraith camp is very easy one alpha + a couple reg attacks if needed. Helps ALOT in the end getting Gear done.

PROS
ZOMG WTF dmg
2-3 hit any squishy if built to any extent
Cant die while pleasuring your self with W(Meditate)
Very fun to play

Cons
Disables HURT OUCH!!

Not really arguable Other than possibly bringing Tele instead of Flash BUT flash is great for a sneak attack on a low health Lane opponent.

In this section i'll be putting everything I've said above together and provide you guys with some hints and tips.

Early Game (lv 1-6)

Get a Doran's ring and head to your lane. During the laning phase you should play a bit aggressive because Using Alpha Strike you can not only farm minions but harrass the enemy champion from a distance.
When the champion approaches closer, and depending on your positioning and which minion you selected, it is possible to predict if your alpha strike will or will not strike the enemy champ along the way. It does take some practice, but once you got it down, you'll see their hp demolished bar by bar.
Once you have enough gold, head back and get Kages pick+ lvl 1 boots, although if you're confident enough you may stay in the lane.

Midgame

During midgame, people will probably start ganking. As yi, it is easy to survive these ganks because you have ghost, ult, and meditate. At this time, you should also be getting blue buff as often as you can to reduce cooldown and increase mp regen. If you want to gank you can also get red buff for the slow. Remember, farm farm farm and get that gold you need. After Rabadon's you should be able to dish out almost 500 damage per alpha strike which is actually a ton, especially to squishies. Continue to play in lane/ganking/alpha striking jungle creeps for extra gold. You should be pushing a bit by this point as well.

Lategame

At this point you SHOULD be king of farm, and you should have your core items. Your burst damage is amazing, and you literally have double your hp since meditate heals 100% of your hp back. In teamfights, your alpha strike hits 4 targets and can take off half the enemies hp. Combined with lich bane, you will literally 2 shot the enemy. All the cooldowns and your ult should be able to let you continuously spam and spam.

Tips and extra hints
Farm, farm, farm. You need it.
Harrass with alpha strike, it's pretty effective.
When you're low on hp and the enemy is chasing you, don't be hesitant to meditate. Most of the time, they'll focus you and pop every spell they have on you. With the armor and mres added, it'll do nothing, and then you can run away.
Occassionally, get a elixer of brilliance for the CDR and extra AP.
Just because you used Alpha strike doesn't mean you can't do anything else. Yi has one of the best base damages in game, combined with his ult and wuju style, he can still pack a punch.
Alpha strike is able to kill off wraiths, wolves, and later double golems in one shot, so use that to your advantage.
Have some fun, take some risks, but don't be ******ed.

I often end up with 400+ CS if im paying attention to farming alot.

100 at 12-15 min is decent higher is better

Pro farming tips

ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA ALPHA

AP YI WINS GAMES SOLO!!!!

I will try to get a full page of wins to add