Master Yi is among my favorite champions to play on ARAM. Though I enjoy him as an AP mid in summoner's rift, The Proving Grounds really brings out the best aspects of AP Yi - a big nuke, devastating combos, and near-infinite sustain.
AP Yi takes some skill and timing to pull off in ARAM - he's a very situational caster, and has to have a team who knows how to initiate around his fighting style. He works very well when you play him right, though, because he's good at cleaning up fights and getting aces, which allow your team to push down towers and eventually win. See the "Strategy" section below for an explanation of AP Yi's fighting style.
If you have any comments, anything to add, or any questions, please just leave a comment. Thanks for reading my guide!
Flat AP Quintessences - the extra AP at the beginning really helps, especially since you should start off by buying a catalyst and thus won't have any AP from items). These are far and away the best quintessences for AP Yi, in my opinion.
Magic penetration Marks - the magic pen helps your massive damage actually get through. Simply the best choice for most casters.
AP/level Glyphs - the extra late-game boost is what I'm going for here. These could be replaced with flat AP glyphs if you want early game stats, but since AP Yi needs a big nuke late-game to stay relevant in team fights, I like going with the AP/level glyphs.
Flat health Seals - these are included to help Yi survive the enemy team's early game burst. Yi has limited durability within a fight, since his sustain comes from his long-cooldown heal, and so a little extra HP will help you get out of sticky situations in one piece. These could be replaced with flat armor or magic resist Seals, depending on the composition of the enemy team.
I got 21/9/0 - All AP in the offensive tree, in order to maximize damage, and just enough in the defensive tree to get that bonus health for a little extra durability. Since AP Yi is basically just a nuke/combo machine, I don't think any other good set of masteries to use.
I always start with a Catalyst the Protector. This is how I play almost all mana-using casters on the proving grounds, and it is great on AP Yi. It keeps him in lane forever due to the great mana regeneration, and makes him beefy enough that he won't die in an instant if he gets caught out of position or hit with some crowd control. You should have enough at the beginning to also buy a health potion as well, to heal up in a pinch when your W is off cooldown.
Build a Rod of Ages as quickly as you can. The earlier you get this, the better. The AP is good, the mana is good, and the health gives you enough durability to turret dive a little and get those kills that would otherwise get away.
Build Sorcerer's Shoes next. These are a no-brainer; the magic penetration is essential on any AP caster.
Rabadon's Deathcap and Lich Bane are the only other two items that I always build on AP Yi. The other two items slots are for situational items. Here are the things that I would consider filling them with:
Abyssal Scepter - The range on Abyssal Scepter's debuff has been reduced, which makes it less viable for most casters, but not AP Yi. When you alpha strike, you'll be bouncing around in the middle of the enemy team, dealing tons of damage and reducing their magic resist to boot. I'd build this item if your team is AP heavy (so that the reduced magic resist benefits your whole team's damage), the enemy team is AP heavy (making full use of the magic resistance), or both.
Zhonya's Hourglass - This item gives you a bunch of AP, some armor, and can save you if you get caught out of position (and can sometimes give you just enough time to get an extra Alpha Strike off, which can have a big impact on a fight.)
Void Staff - I go with this item when the other team is stacking magic resist and/or the rest of my team is mostly doing physical rather than magic damage (if my team is mostly doing magic damage I'll get Abyssal Scepter instead, so that we can all benefit from the debuff.)
Rylai's Crystal Scepter - This is a good item if you find yourself being burst down very quickly, as it gives you a little bit more HP and allows you to run away from the enemy more easily (due to the slow) if you get out of position after a misplaces alpha strike.
Deathfire Grasp - This item is good if your team needs a little more damage to get teamfights to the point where your Alpha Strike/ Highlander combo can go off. Buying Kage's Lucky Pick early (on your first back, ideally) and finishing Deathfire Grasp late will give you a gold advantage, as well. I personally rarely build this, but have seen it used to great effect on AP Yi. The cooldown reduction will also help you poke with Alpha Strike more often.
Morello's Evil Tome - I get this item if the opposing team has a champion who gains a lot of life quickly during fights ( Swain, Rengar, Fiddlesticks, Master Yi, Dr. Mundo, etc.) since the active shuts down life-gain effects in a big way. The AP and cooldown reduction are also good for AP Yi.
The Proving Grounds
as AP Yi involves two distinct modes of play - I call them the "poke" and "dunk" phases of the game.
The Poke Phase
Early on, most of what you'll want to do is try to poke at the other team to whittle them down until your team can pick some of them off. The trick to doing this is to wait until there are only one or two minions in a wave left and there is at least one enemy near them, and then use Alpha Strike, targeting a minion. You'll bounce around, get some damage on the enemy, and end up safely back at the minion. You can extend pretty far, and as long as you don't die you can just heal up all of your health with Meditate. You can also use Alpha Strike to clear minion waves, but you will be unlikely to get any damage on the enemy champions if you do this, so I try to avoid it unless the minions have pushed dangerously close to my own tower.
The Dunk Phase
Mid- and late-game are when AP Yi can pull off some pretty terrifying combos. When a team fight breaks out, your job is to stay in a safe position with your abilities on cooldown until you spot somebody who is close to death. The you trigger Highlander, run up to your target, and Alpha Strike onto them. When you land, they will die (or should be a single auto-attack/ Lich Bane proc away from death,) which will reset all of your cooldowns. If somebody else is low enough, repeat this whole process. If there is another low target, you can get another kill, at the same time putting damage on any champions that are hanging around. This combo works best when there are few minions around and the enemy team is bunched up, and will often lead to double-kills, triple-kills, or more.
The downside of this playstyle is that if you can't pull off this combo, you are basically useless. It takes a lot of timing and practice to make sure you have your abilities off-cooldown at the right times and to pull off the combo consistently, as well as a team that can front-load enough damage for you to start getting kills. Besides this, hard CC in the middle of your combo will shut it down (which is why Cleanse works well on AP Yi.) If you get stuck, and there is no CC around to threaten you, you can tank an incredible amount of damage for 5 seconds by using Meditate in the middle of the enemy team.
Meditate is useful in the dunk phase for tanking enemies (if they don't have any hard CC to interrupt it), recovering health after a fight, and tanking towers (post-mid-game, you'll be able to tank a tower while your teammates bring it down and completely heal yourself with one trigger of this ability.)
Wuju Style is occasionally useful in teamfights if your Alpha Strike did not manage to kill your target and you need the extra damage from a Lich Bane proc.
A note on teamwork: if your teammates don't seem to engage in a way that works for you, tell them what your plan is. I've had people who wouldn't initiate because they thought that I was hanging back when teamfights started because I didn't want to engage or wouldn't help. Usually if you explain to people that the rest of the team needs to get somebody low before you can do your thing, they will adapt their play style a little bit to give you some openings.
Similarly, if you don't do well as AP Yi, don't just blame your teammates. He's hard to play well if you don't have a team composition that can support hi, and flounders against teams with a lot of CC and very tanky teams.