Master Yi is a terror at early levels because he is cheap, easy to play ,and deals a ton of damage. Most early game players dont know much difference between AD and AP so they build straight damage and attack speed. This makes Yi VERY squishy and susceptible to items like frozen heart, ninja tabis, and thornmail. But the best way to stop him is with a stun. stun him and he will melt like butter. Jax destroys Yi and is fairly easy to play. AP Yi is a little different. Use Banshees Veil to block his Alpha Strike, then stun him and drop him. He might get away and meditate but thats OK because with a banshees veil he will never hurt you.