Master Yi Fast Crit slayer!

So I'll just give you a quick guide )

So, for masteries I go for 21/0/9, you could also go 21/9/0, but I suggest 21/0/9, because it gives you more time on your lizard buff.

For Masteries I go for Ghost, which helps you chase down some1 who is fast, and exhaust.

For Runes, Armor Pen Marks, Armor Pen Quints, CDR glyphs, and Dodge/Mana regen seals.

For skilling order, start with an alpha strike, and max out wuju style, and after maxing out wuju style, it depends on the enemy team, if they are harassing a lot, get meditate, if you are doing the harassing, get more alpha strike.

Item build, start with a doran's shield, and go for berseker's greaves, after this, rush ghostblade, and after ghostblade, go for a phantom dancer. By now you have 47% crit chance, and about 150 dmg, so you can do a lot of damage fast. At this point, I personally get 2 cloaks, 1 for another phantom, and one for Infinity edge. I sell doran, and get whichever component I can of infinity edge, because the unique passive on IE gives me more damage, and the high crit chance guarantee's me double damage on each strike, while the high attack speed gives helps me gank in seconds. After finishing the second phantom, the boots become unnecessary, thus i sell them as well, and look, you still have 2 spots left! I usually get Bloodthirster for lifesteal, and black cleaver for it's unique passive. At this point the game should end!

good Luck )