Ansem: Use a Keyblade that increases your MP, if you feel the need. I generally use the Three Wishes, myself. It's ability to always end combos with a critical hit is divine. Also, the Ray of Light and White Fang accesories on Sora could also be helpful.

Kounetsu: Keep Donald in your party for the first battle. You can switch to Goofy afterwards, but I always let Donald tear it up with Thunder.