Do you have trouble with the last Riku fight? In order to defeat him,be around level 70 and have about 5 "0" cards and whenever he uses the move where the room gets dark and he jumps off the walls(Dark Aura) or Dark Firaga, use them. It is important that you keep them at the end of your deck so you wont have to search your deck for them. Once you defeat him, go to destiny islands and save it and then go to Castle Oblivion and you face Larxene.