Master Yi is one of the best character in the game in my opinion.Here is one of the builds i use with a help from a friend he helped test it and it seemed to be working very good. When u start make sure u buy everything correctly and in order. You will have at least 3k hp by the time u get warmhogs at lvl 18 or some what around that lvl and with ur Life Steal you should be able to kill a lot of people.
Other items are also good too. For one u can get a Quicksilver Sash if the other team has a teemo or someone that bind or stun. Quicksilver Sash is a very useful item if needed.
In a team battle u can rush in there and take out probably the whole team out if ur careful and know what ur doing. For one U have lots of hp and lots for life steal. Life Steal Damage and Hp= invincible. Just remember if u have a person that can stun or blind make sure if u have Quicksilver Sash use it.