Luxord was easy, just flunk the game, become a card, jump around for a while, become sora again, and kick his ass.
Xaldin was just a bit annoying, but easy once you figured out the patern.
Demyx in the second batlle was the hardest of the game, only time I actually lost. But that was just the first time, when I played the game again I beat him in one try. Just keep on hitting him and stay close, so he can't summon the water thingies. He didn't do it once the second time I fought him. If he doesn't summon them, you practically win.