I have played master yi a lot, however, usually when I play him I win so I try not to play him as much (it gets boring). Here I'll give you a short and sweet build to ensure success. First off buy beserker greaves and hp potsx3, then start leveling up. You want to put most of your points into wuju style until lvl 9 and also the q attack which i forgot the name of. Then once you can buy 3 lifesteal scepters, you should have enough money by lvl 7-8 for this if you're playing correctly. At this point you can start jungling, usually I head to dragon first and kill it. It only takes out 3/4 of my hp max, if you feel the need use your ultimate or bring a hp potion along. Make sure to keep minions pushed to but keep jungling. Soon you'll be OPed enough to rack up kills. Since by now you have beserker greaves and 3x life scepter staffs, buy whatever according to the enemy team. Remember, all characters should be balanced, meaning if they have armor buy armor penetration, if they have health buy medreds bloodrazor (sp??). I know this guide isn't complete but you CAN KILL DRAGON AT LVL 7 OR 8 and can ensure success later on. This build is meant for more of 3v3 than 5v5, make sure that if your opponent gets thornmail to buy some hp, like frozen mallet or warmogs armor as if they can tank and dish out dps then you will die without more hp. Critizism will be appreciated, and duly noted. Take care :)