So this is my first build here so hello everyone!
I have played with master yi many times and i know that there is a LOT of different builds out there. So this is just one of them. I find this build so good not depending on the champs againts you. AP or AD. Go and give this build a try! Youl love it.

I dont know so much about the runes but armon ben, dodge, crit dmg and more armor ben is good for yi and when they see your dmg they are going to build themselfs dome armor.

Basic dms masteries 21-9-0 with ignite and exhaust. Sais all.

So this is where the build happens! Everyone know that Yi is low healt champ that would die in a sec if the team is going to focus you. BUT with this build there is no such big worry when the team is focusing you. You are going to have almoust 4000hp at the end of the game with the warmogs at full. Basic start with boots dorans blade for extra healt and some dmg. Go base as soon you get 920 gold for boots. Then rush for brutalizer. Nice cooldown reduction and dmg. Then the fun part. Try to get some kills at this time and go for warmogs. After you have that just farm and farm so that you get you warmogs full. After that its time to finish youmoo's. Youl have dmg now and alot of hp. Ok now its the dmg time. When you get your atmas impaler youl notice that your dmg is going to ingrease a LOT. So now you can go to 1vs2 fights easily. With youmoo's and impaler you are going to crit more so the next item is infinity edge. More crit dmg and dmg output. Last item is Frozen mallet to add MORE hp and slowing the fleeing champs.