How do you defeat Demyx in Kingdom Hearts 2?

I'm in Hollow Bastion, and you have to defeat Demyx before you have to fight the 1000 Heartless, and I defeated him the first time in the Underworld with no problem, but now it's harder for some reason. I'm on the beginner level. I'm a level 33 and for some reason I can't defeat him, whereas I always have been able to defeat him before. Help :(