I wouldn't use up any magic if I were you, unless you were gonna pull of Ars Arcanum, but that rarely gets in all the hits it allows, or when you have to Cure yourself.
Let me see If I can explain this one. When you first start off, you'll have to hit him 20 Times to get his bar moving. He'll do basic sword attacks and his pillar of fire is easy to dodge. Guard when you have to, give a combo, and he'll teleport. If you're really Gun-ho, go in and strike him nonstop, but that wouldn't be wise against him.
When you get him to the orange bar, he'll start doing hit and run attacks. Use Guard to the best of your abilities. There's always a sign before he does Sin Harvest. Sometimes, he'll run right at you, and teleport and do Sin Harvest right behind you. He gets serious during this mode too. The screen will get dark and he'll yell, "Prepare Yourself!" Then he'll do some amazing looking combos that can be dodged easily if you use guard. Trust me, Guard is your best friend in this fight.
Once he's in the yellow or green, get ready. He'll turn Dark Green and have meteors send shockwaves to the ground, and you have to be careful not to be hit by them. After that, there'll be a Huge Explosion, but DO NOT GO AFTER HIM. He'll teleport and do Sin Harvest every time after this attack, and if you aren't ready for it, it's over. In this mode, he'll also, turn a purple color and make Dark Orbs explode around you. The best bet is to Dodge Roll. His combos will increase in power, but don't worry, you can still hit him. Always have your Guard up, because you have no clue where he'll pop up next.
All in all, he isn't that hard unless you know his strategy like I do. Using any weapon other than the Ultima Weapon will make the battle longer. The Ultima Weapon is the strongest Keyblade in the game and will guarantee victory.