Just equip Restore guard and a number of Cure/Cura/Curaga. Use combos and shotlocks, and always watch your health. In the second part of the fight you just have to fill the limit bar prompted under "Vanitas", at the lower left part of the screen. Just throw shotlocks or attack and when your attack collides with his turn the direction stick counter-clockwise as prompted on-screen. When the limit bar is full just press X to end the fight. [video=youtube;1Y8SstMQ4pQ]http://www.youtube.com/watch?v=1Y8SstMQ4pQ&feature=fvsr[/video]
I agree with Patman. If you feel the need to use offensive spells make them quick casts (for example, firaga, thundaga, etc.) The more advanced spells will often be the death of you because they take longer and Vanitas can teleport-recover, leaving Once More useless and you unable to heal. The exception being Faith/Salvation since they aren't too long and heal you at the end, just make sure you have a nice health cusion when you begin the cast with them.
