How do you fight terra in Kingdom Hearts 2 final mix

I own Kingdom Hearts 2: FinalMix+ and you can fight Terra. He is an optional boss and he is unlocked only after you have beaten every other world. After you have beaten the other worlds, return to the Disney Castle and go to the Throne Room (The huuuge hallway) and enter the small room at the other end of it. Here there should be a new crack in the wall where you will have the option to fight Terra (where it will teleport you to a new area for the boss battle). Happy playing ^.^