Kingdom hearts, fighting xemnas as Riku?

When you have to be Riku after Xemnas traps him in some weird magic thing, hwo do you get to him, his double keeps blocking you with a shield and that takes away most of your life, i try attacking him but he keeps blcoking himself and Riku dies, it's really annoying, Do you actually have to defeat the double? or get past him, I don't know what I have to do :s