Well, is it the one you have your party members or going it alone? I had trouble with the latter one too, but all I can recommend is plentiful potions, more training, and dodge roll. When he's doing that ram thing over and over again it's easier to evade when using dodge roll. Always back up after a combo, he will try to do an attack. You can usually tell when he's letting his guard down, buy you'll have to go with your gut on when to stop laying down hits.
In the beginning of KH I fight Wakka and go far away and play baseball, You can get lots of XP and he doesn't get hurt!!
