Please leave why you downed this or whatever haha

Please note that I main Annie, so this is meant for mages to be able to play master yi.

As a Master Yi you think that you should be getting all the kills and carrying; This build will allow you to carry as well as play as a mage. The items above show the order you SHOULD play as Yi. If a better way works, try it out, but I believe this is the best order for an AP AS hybrid.

I chose a 21-0-9 masteries build to fully max out your output for damage, as well as getting some buff for your mana pool. The mana regen is probably the best thing for an early game Ap yi.

Summoner spells:
I chose Flash and Ignite as that is what i use for Annie. Flash for the all around mobility, and Ignite for early game kills.
Other Good ones are:
Ghost
Cleanse
Exhaust

The bad ones are:
Revive
Rally

All the other ones are neutral, and as you play, you should get a good idea of what you like to use.
The traditional Yi uses a:
Infinity edge: dmg output
Phantom Dancer: the speed for your damage
Bloodthirster: lifesteal
Your moms ghost blade: hybrid attack speed crit
Berzerkers: Movement and more AS
Sword of the Occult: for the extra ouch

As you notice, the traditional yi does not have any survival items such as health or armor or magic resist.
Thesse are the replacements that this build provides you so you get all the dmg output in one short burst, as well as some of the survival items that does not come with the traditional yi:
Malady: AS and ap for output
Rod of Ages: for the AP, health, and mana boost.
Archangels staff: If you've played AP yi before, you might notice that he is very mana hungry all the time. With the help of Archangels staff, you can build up your mana pool as well as have the crazy ap.
Hextech: The traditional yi survives by using a lot of lifesteal to regenerate health with hits. AP yi does not contain any regeneration of health so Spell vamp provided by Hextech will help, and it is a hybrid item, so it also buffs your AD damage; the Active is nice too to initiate.
Zhonyas Ring: 25%+ 120 AP. Truely amazing for any AP yi build.

Situitaional items:
Mejai's Soulstealer: Provides an extra buff for the ap. 160 ap at max plus 15% CDR.
Sword of the Occult: Provides an extra buff for the AD. 100 AD at max plus 15% movement speed.
Note*: These items should only be used with an early good start. These items will only be strong if you can rack up a lot of kills & assists.
Manamune: If you feel that your damage output is not high enough for Yi, Manamune is a good viable item as it buffs your AD the way archangels staff does. However, you can not stack it, so make sure you only buy one haha.

By the end, your core should be:
Malady
Boots of your choice
Archangels Staff
Zhonyas

How to play as AP yi:

To play AP yi, you need to be smart. You should be able use Alpha Strike whenever a teamfight happens, so make sure you keep that out.
The AP ratio on ALpha Strike is 1:1 so it makes it extremely viable to play AP yi
The AP ratio on Meditate is 1:1.5, as well as the armor buff you get, so it makes you also a very good tanky character when you care taking heavy damage.
Wuju style has no AP ratio, but it is a very good move to keep you strong throughout your cooldown times.
Highlander allows you to spam Alpha strike as much as possible. So turn it on when youre about to kill to guarantee the cooldown refresh that it provides.
Your passive should be used right after you turn on Wuju style to max out damage output on your AD side. I would suggest you play with someone that has CC in your lane, so you can target the champion directly instead of having to gamble for the hit on alpha strike when you target the minion.

Early game: You want to spam alpha strike as much as possible, as it is the main form of harrassment for you. You might even want a friend or someone to get blue buff for you so you have a higher mana regen and a lower cooldown.

Mid game: Ready to shine? You should have your malady and your boots by now. Your main method for kills is this combo:
Alpha strike
Enable Wuju Style
Deal Damage and take Damage
Meditate
Highlander
kill
Rinse and Repeat

Late game: Pop in alpha strike after your tank initiates, and get ready to chop chop chop. Do the same thing as you did for Mid game, but now, you will be much more powerful with your 500+ ap. Dont be afraid to ks, as it brings so much more for your team overall if you can repeatedly spam Alpha Strike, Meditate, and Highlander for the kill. Rinse and Repeat.

Have fun with AP AS yi, as I believe he is extremely fun to play, way more than the traditional yi.

I thought I might add something more so I can tell you more about AP yi. Your highlander refreshes all cooldowns, but it does not refresh your mana pool. So make sure you keep up your mana pool or else you would be useless. Also note that Gunsoo's Rageblade is also a very good item for AP AS yi as it builds up more ap and AD as you strike, and plus, the name is soooooo cool. Most of all, build items situationally as it does much more if your realize how to play accordingly to that particular game and base how you play on your items. If you choose Occult over Mejais, obviously you would want to hit more with your right cick attack. If you choose Mejais over Occult, your spamming move will be used much more; Make sure you know what you're doing. As you learn how to play situationally more, you would learn how to attack and kill much faster, and be called "beast yi" or the like. Most of all, remember to have fun and to kick some ***!