Ok, for this build, it will most likely bring you to victory, every time. You should only aim for this build if you get fed early on in the game, or else you'll be struggling the whole game to get the items. Yi does a lot of damage, but he's very fragile, squishy, like glass. Meaning stuns screw you, pretty much. Since yi can't be slowed with his ult, the only champions you should cautiously fight are the ones with stuns. I've done so many games with this build and have won so many. It's crazy. Not too recently i went 15/0 :)

For the runes, the armor penetration marks are just that small bit of help for you to get the tanks down. I've brought down rammus' that have thornmail before, easily with this build.

For the attack speed Seals, it helps throughout the whole game honestly. Yi has natural damage, which is why he needs attack speed early on.

The Quintessences (fck that word) and glyphs are for the only purpose that crits help SO MUCH with life steal, and you will have something very close to 100% crit chance by the end of the match.

Ok, you start off with a dagger, giving you the tiny bit of extra attack speed that will probably help you get first blood easily, along with a health potion in case you ever need it or don't feel like recalling. Then, you finish up the berserker's greaves(atk speed boots). Then you get phantom dancer (attack speed). Definitely start off with Zeal though, then get whatever your money can buy: Either the cloak or the dagger (or both :D). Then comes the fun part, your first bloodthirster! :D Make sure that if you die, you re-stack the bloodthirsters with either minion kills or neutral monsters. 1 stack isn't much, but 40 helps a lot. You need the extra life-steal and damage, if you want to be able to kill a fully-eqipped rammus :) After the first bloodthirster, it's really easy. You just get the items as you go, Inf edge, then two more bloodthirsters. And uh, ya. You stack them, then when you have a little over 2200 gold, sell your boots and buy another phantom dancer. Both of them together grant enough movement speed to match(or exceed) the speed you have with boots and a phantom dancer. The last phantom dancer is needed for the extra 30% attack speed and crit chance. After that, just buy all the consumable elixirs.

Ok, all you really need is Yi's E, his attack damage. This whole build is based on attack and life-steal, so his W doesn't do ****, honestly. You use the Q to pull yourself in, then you ult and E as you follow them to their death, or they'll be ******ed and try to kill you. You just about always will get a kill after the first bloodthirster is bought. Of course, unless they have a stun ^^;

Whichever you're more comfortable with, you can either use Flash or Ghost, just note that Ghost isn't always for running away. And since Yi doesn't have a stun, Flashing to someone that's using Ghost would do much if they get away. So most likely, Ghost is better.

If not ghost or flash, cleanse would be good, maybe even better. It just depends on your play-style. If you're used to having cleanse, get cleanse. If you're used to flash, get flash. And the same for any other spell. Just note that other than Ghost, Flash, Cleanse, and Exhaust, most will do you no good.

Exhaust is for one single thought: 1x1 battles. With yi, you'll be trying to get kills left and right, so if you go one on one against someone with exhaust, you're gonna want to be able to fight them back with fair rights.

Ok, this is the final thing for this build. This build is based on life-steal, and armor penetration runes. If you're facing someone with high damage and life-steal like you, and they have thornmail, make sure you're near their minions, or something non-champion to attack. Life steal them, then go back to fighting the champ. You'll have full health while they just sit there attacking you. Same goes for a mordekaiser or rammus, just attack a good deal of minions real quick and go back to fighting them. This just about always makes you win, unless you've died recently and don't have your bloodthirsters stacked. Or if you have to run away to fight minions, and they're not right next to you, the enemy champ could deal damage to you on your way and most likely kill you, since Yi is like glass -_-