How do you play riku in kingdom hearts Chain of Memories?

when sora gets trapped by final xemnas you have to use riku to free him, that copy that xemnas does is quite frustrating you need to attack him or he will push you back to the start and you will not be able to save sora,when you get close to the real xemnas a RC(reaction command) will appear on the command box JUST PRESS IT and done