In the start you should play defensive since Yi is a very weak champion in early game. Dont waste all your mana on useless spells like Alpha Strike, use it when you know you can damage the enemy or you know you wont get hit by them in any way. A Supporter would be a big advantage such as: Soraka, Alistar, Taric and Shen. Use your potions wisely and dont play offensive. Late game Yi is still kind of weak and that is why he needs a supporter. But he is fast and srong late game, so he can pick up kills very easy but also die very easy, since Yi is an Assassin he is good at ganking others lanes. You dont have to buy The Black Clever, if youre playing against a tank buy The Last Whisper.