I *just* beat the bugger. Once you know how to do it, he's shockingly easy.
1.) I cannot stress this enough. PLAY CONSERVATIVELY! Heal before rushing in if there's even an INKLING of you being seriously hurt.
2.) Watch for action commands. It's always preceeded by the screen getting dark. This sounds like a no duh thing, but it caught me off guard once or twice, and that attack HURTS!
3.) He's pretty freakin' vulnerable after that attack. For the love of god, take advantage of it.
4.) I dunno what the move is....Horizontal Slash? Whatever it is, I'm sure it was made for this battle.When Sephy launches you, use it when he comes in for attack #2, and you can completely turn the combo around on him.
5.) If he starts rising TAKE! HIM! DOWN! One attack will interrupt this attack, but if he is able to fire it off......biiiiig trouble.
6.) It's important enough to warrant being mentioned twice, basically. Cure, cure, cure. Potions, spells, whatever it takes, keep your health up.
7.) I dunno about Second Chance, but Once More saved my sorry ass a few times. USE IT!

Basically this whole battle is common sense. You still wanna be a high-ish level (I was 79) but in comparison to the first KH's Sephy...? Meh.