MASTER YI!

First off, I'd like to say he's one of my favorite champs to play. I build him AP 95% of the time. I think its a lot more fun than just auto attacking people to death, and I believe he's more useful to a team in this role.

For my summoner spells i use FLASH and IGNITE.

Other possible combos:

GHOST and FLASH - I don't find GHOST to be particularly useful with Yi since Highlander gives him so much movement speed.

FLASH and EXHASUT - A good combo. If my team has plenty of IGNITE I may very well take EXHAUST, it's a situational thing.

FLASH and CLARITY - Clarity is just a bad spell for Yi with this build. With the Rune/Mastery set up, along with your starter item, you should not need it.

FLASH and CLEANSE - This is the best combo for survivability. I much prefer CLEANSE over GHOST especially when facing a team with high amounts of CC. Keep that it in mind.

TELEPORT - Its a viable option. May be good if you have a Teemo on your team. Good for ganks, For Ex: teleporting to a shroom, or if you have wards in their jungle at buffs etc. and can teleport to them. This isnt really a backdoor build so it wouldn't be good to get it JUST for that. And with your Meditate healing you for so much, there's no real reason for you to go back to base during the laning phase unless your lane is pushed or have someone covering for you.

Stay away from the other summoner spells.

Your runes and masteries should focus on early game dominance. They should give you the edge you need to kill your opponent when level is the same, and you both have the same amount of items in gold. Keep that in mind when deciding what you want to use.

Quints:
Greater Quintessence of Fortitude (+26 Health) My #1 Choice. They really help Yi take some damage early game. As AP Yi you will constantly be alpha striking to the opponent putting you in range of their skills. So it is important to have enough health to take the damage, get away, then meditate to heal yourself, and be ready for more.
Greater Quintessence of Insight (+1.89 Magic Penetration) Not a bad choice for a LITTLE more damage (emphasis on little). I really think these are more helpful late game, but if you need mag pen by then you can get items for it.
Greater Quintessence of Swiftness (+1.5% Movement Speed) I like these a lot. One of my favorite Quints in the game. Not a bad choice.

Marks:
Greater Mark of Aclarity (+1.7% Attack Speed) Great for early game. Your alpha strike is on a pretty high cooldown early game. So it's nice to utilize a bit of extra attack speed for that OOOONNNEEEE HIT that would kill a champ before they get back to the safety of their turret.
Greater Mark of Insight (+.95 Magic Penetration) These would be my second choice. More damage, but don't think your gonna be one hitting your enemy early game because you got these marks.

Seals:
Greater Seal of Replenishment (+.41 Mana Regen /5s) No replacing these. You have to have the mana regen for your early game.

Glyphs:
Greater Glyph of Potency (+.99 Ability Power) I use these 100% of the time on AP Yi. Gives you that extra damage to dominate your opponents early game. This is more so than with most AP champs, because his ratio on Alpha Strike is 1-1, and his Meditate is 1-1.65! Nuff said.
Greater Glyph of Focus (+.65% Cooldown Reduction) These are viable runes on AP Yi. However, I think they are more useful to you mid to late game, and I'm going for early game performance with my rune set up.

I do a pretty standard 9-0-21 set up on AP Yi.

Utility Tree:

Perserverence - 3 points for the much needed mana regen.

Good Hands - 1 point for the simple fact I don't use ghost or teleport with AP Yi. If you decide to make sure you allocate the point accordingly.

Awareness - 4 points. AP Yi is very good in middle lane and solo so the extra experience is always needed in those situations. Also, He does not have very high base mana so there is no real reason to put points in Expanded Mind.

Meditation - 3 points for more mana regen! How can you say no?

Utility Mastery - 2 points, you will be grabbing blue regularly! this is so important.

Quickness - 3 points. You can never have enough movement speed. It also synergizes with his ult nicely.

Blink of an Eye - Do you really need an explanation?

Intelligence - 3 points. With this and Sorcery from the offense tree you will total 9% CDR right off the bat.

Presence of the Master - So pro. You will FLASH around more than anyone.


Offense Tree:

Archmage's Savy - 3 points. More ability power! need i say more?

Cripple - 1 point. There is no reason to put a point into Plentiful Bounty or get and extra .66% crit chance from Deadliness. This aint DPS build! From time to time i do take exhaust instead of ignite. This is purely based on the summoner spells my team has and what we are up against.

Sorcery - 4 points. Goes great with your cooldown reduction in the Utility Tree and you have to have it for Archaic Knowledge.

Archaic Knowledge - 1 point. 15% Magic Penetration. BOOM!

This is pretty basic stuff.
Alpha Strike: This is TOP priority. It's your main damage, harass, and farm skill. I take a point in it every chance I get.
Meditate: 2nd priority. Take a point into this at Lv. 2 and begin maxing it after Alpha Strike is Lv. 5
Wuju Style: Last on the list. You DO want to take 1 point into this at Lv. 4, but leave it at that till end game.
Highlander: Of course you will put a point into this at Lv. 6,11,and 16


This site has misinformation on one of Yi's skills so refer to the link below for EXACT descriptions and ratios.

Master Yi's Abilities Descriptions

Dorans Ring: This is one of the few items for AP Yi that you should ALWAYS get no matter what. It's your first item and it's a MUST!!!
Fiendish Codex: This is also an item you should ALWAYS get. Why? Because it builds into a multitude of items that are situational and always have a use with this build for different types of opponents. For Ex: If you are facing a Tank heavy team, Deathfire Grasp is a great item. Or, if you desperately need to backdoor because your team is failing hard and its the only option you see Nashor's Tooth will do the job.
Morello's Evil Tome: One of my CORE items with AP Yi. For the price and benefit that it gives you there no real comparison. Sure Nashor's Tooth gives you a lot of attack speed but its more expensive and has 20 less AP. While Deathfire Grasp is good vs. tanks, if their team only has one I really don't see a point in getting it.
Sorcerer's Boots: Boots with this build are interchangeable. I like Sorcerer's boots most of the time. Merc Treads are ok with a HEAVY CC team. Ionian Boots of Lucidity will give you max CDR with this build. So as you can see there are many avenues to go. Buy smart, take into account the people you are fighting, and the teammates you have. (If you have a Morgana shielding you as you initiate you wont need those Merc Treads will you :P)
Rabadon's Deathcap: CORE ITEM. You gotta have it. It gives you too much AP too pass up. And AP with Yi isnt just damage its tankiness because of his Meditate skill.
Rylai's Crystal Scepter orRod of Ages: I like to get one of the two. Especially against high burst teams. If you can survive the initial burst after you Alpha into them. They wont be able to kill you when you Meditate. 90% of the time i go with Rylai's because Rod of Ages is and early game item. (It gets stronger as time passes) By this point into the game if you have been getting kills and farming it will be 30+ minutes into the game. SO it would take 20 more minutes for Rod of Ages to reach its full potential. Which is the 50min mark. (Games normally don't last that long)
Zhonya's Hourglass: Great item for AP Yi!! he gets OP at this point. He has Meditate which gives him 300 mres and armor plus this defense. THEN while he's channeling he's healing for over 2k hp over 5s. ANDDDDDD You are invincible for 2s with the active. SO BEAST!
Abyssal Scepter: I like this Item because it gives good AP + mres and has an aura that reduces champions mres by 20. So it not only gives you more survivablilty, but it helps your team as well.


CORE ITEMS:

(NOTE: I do not have boots in the core items because there are MULTIPLE viable options for boots with this build. It DOES NOT mean you don't need some type of boots.)
Doran's Ring
Morello's Evil Tome orDeathfire Grasp
Rabadon's Deathcap
Rylai's Crystal Scepter
Zhonya's Hourglass


REMEMBER - There are always situations you aren't expecting. Players on the other team may get fed. You may need some defensive items. You may have underestimated the other teams CC, and didn't grab CLEANSE so its becoming a problem. Don't be afraid to grab a Banshee's Veil in that situation, or grab the Chain Vest for Zhonya's early in the game if you are receiving too much physical damage. Buy smart. This is a guide! and should be used as such!

Pros:

- Great creep farmer.
- Awesome lane sustainability with Meditation.
- Excellent damage.
- Will carry.
- Can chase down champs like no other.
- Good escapability at Lv. 6+
- Extremely fun to play.

Cons:

- Vulnerable to certain types of CC.
- Alpha Strike is his only high damage skill

Laning:

The early laning phase is AP Yi's stomping ground. He can really dominate here and cause lots of frustration to the enemy.

Alpha Strike will hit up to 4 targets. You can use this to your advantage by attacking a minion close to the player extending the range of the skill. It's important to know that it DOES NOT really prioritize champions over minions. So, if you attack a minion and there is 10 standing around a champion it is very possible that you are not gonna hit him. When using Alpha Strike in this manner, do it when there are 3 or less minions around.

When attacking a champion head on with Alpha Strike make sure you have most of your HP. It is going to teleport you right on top of them. A good player is going to run BACK when he sees you have used the skill, and will pull you closer to the turret than you may have expected, or next to his lane partner. Just be cautious of that if you are solo or mid, and if you are laning with a person make sure they know you are about to dive in.

At Lv. 2 you have your Meditate and Alpha Strike. Your job is to non stop harass. The good part about this is you will be farming and harassing all at the same time since Alpha Strike hits multiple targets. If you take a good amount of damage you quickly run back and Meditate to full, or near full health. Because of your mana regen set up, you have unlimited sustainability with Meditation. Keep at it till the Champ or Champs are low.

Now if they have support in the lane or good HP regen it may take you a few more levels to get a good harass going. Who knows you may have a ****py lane partner! Once you hit 4 you will have WUJU Style. Simply, harass the same way, but as soon as you Alpha Strike activate WUJU and get a couple auto attacks off with the damage bonus.

Even further! Yi's passive allows him to auto attack 2 times for his 7th attack. So watch your passive count. Last hit some minions until it's at 6. Alpha Strike the target, activate WUJU, then auto attack once, and not only are you getting the extra modifier from WUJU Style but the attack is DOUBLED!

Let's say you are mid,and you got the champ low. 1 of 2 things is going to happen now.

1. They are going to B. Which allows you to continue farming gold and experience. You will surely have the advantage.

2. They will stay and turret hug.

This is what I hope for, and also one of the reasons I take IGNITE. In general the target should be at 1/4 HP a little more is fine. If its a low base HP champ like Teemo or something even 1/3 HP is fine. All you do is make sure you are full HP using meditate. Push your minions to the turret. ALPHA -> IGNITE -> FLASH (to safety). Be sure you don't get baited into someone with heal. I doubt you would die, but you would waste FLASH and IGNITE and come away with no kill.

As a MID if you are AP Yi ESPECIALLY at Lv. 6+ you should be ganking top and bottom regularly. Turret diving low champs and escaping with Highlander then using Meditate to heal.


Teamwork:

AP Yi is an awesome team player. Lets run through some scenarios.


Trouble Pushing Towers? AP Yi is deceptively tanky. Once you get 3 or 4 core items you will be able to do some really clever things.

So there's a classic standoff at the enemy teams 2nd middle tower. Make sure a wave of minions is pushed up to the turret as always. You act as the tank. Make sure your team knows to attack the turret and is aware of the plan. Start attacking the turret, once the enemy starts attacking you use your Meditate. That's 5s of tanking time. If your team was attacking the turret the entire time you were tanking the turret should be dead or close to it.

BEWARE of IGNITE EXHAUST and CC as they can really screw this up. You ALWAYS want to have your Ultimate(Highlander) out of cooldown and available when attempting this.


This same tactic can be used when split pushing. Say there is a Vayne that can really kill turrets fast but you haven't been able to push lanes. Wait till the enemy starts pushing any lane as a team. Let your guys know what you are doing so they are prepared to defend with 3. Take Vayne through the jungle to the turret with the least health. Tank the turret and help Vayne attack until you are close to half health. Pop you Meditate and continue tanking while Vayne finishes it off.


Team Fights:

Lets talk about some different roles he can fill.

Initiating - Be sure you have some hard CC on your team if you want to start initiating before the tank does, and you should have Zhonya's Hourglass. Make sure your team is ready and in good position. The enemies position is just as important, because you want to hit max targets with your Alpha Strike. Being that its a teleport of sorts, it's also fairly easy to get past the tank(if they are close together), and Alpha Strike the squishier targets in the back. 9 times out of 10 you will be focused because your YI!!! He's squishy right? WRONG! While channeling Meditate at max level you gain 300 armor and mres plus you will be healing for insane amounts (over 2k hp with 2/3rds of your build). Once they focus you, use Zhonya's active. This should divert their attention to another champion. If you see this happen you can hit your WUJU, start attacking squishys, and using Alpha Strike(by this time their squishys will be dead or running). If they stay on you or refocus you. Use meditate. There is no reason your team shouldn't be able to take out, or make their carries flee in horror after 7s. (Note: If they have lots of stuns, silences, suppress, be very cautious. This will knock you out of your channel and death will be eminent, as always make sure Highlander is up as well in case you need to run)


Clean Up - Some people can call this KSing and cry about stuff like that, but the fact is: Alpha Strike will be doing 1000+ damage. KS my ***. Other champs that u can compare this tactic to would be Fiddle or Katarina. Your job is not to wait out the team fight till the enemy team is low, then jump in after half your team is dead. It's all about positioning. Positioning should be adjacent from your team in some brush or out of sight range. The important thing is not to attack from the same direction as your team. Come in from another angle. Let your tank initiate, be sure that the fight has started and all of the enemy team has committed to it. Now you make your move. Its important you activate your Ultimate(Highlander) BEFORE you Alpha Strike in. This is because all of your cooldowns refresh when you kill a champion while your Ultimate is active. So if you just kill one champ on your first Alpha you can use another immediately after. This is devastating and will definitely keep your team winning.

Well, that's about it for the guide. Any feedback or suggestions are welcomed. Everyone does things a little bit differently and putting all those ideas together is always the most productive approach.

COMMENT AND VOTE IT! Much thanks! PEACE