Here, I was stuck on the same spot as you, so heres some tips:
-Go to lands you've beaten and defeat alot of heartless and nobodies to level up. Hallow Bastion is good, because it's large and has alot of tough enemies.
-With the munny you've earned by leveling up, you should be able to buy some Mega, high and normal potions. Equip them to every party member.
-Make SURE to have Donald's Duck Flare spell equipped. It was so helpful, I defeated Xigbar in five minutes, and we never got to the small platform.
-When and if he puts you on the small platform, run like crazy in a circle motion around the platform, so he's at your heels but doesn't hit you.
-Don't waste your potions on Donald or Goofy. They can heal and fend for themselves.
-The only reason you should heal them is if your health is fine, you ahve many potions left and they are dying.
-Equip SCAN. It's very helpful.
-Ask me for anything else.
Use my tips, and you should easily defeat the pirate-eyed, creepy-scarred man that is Xigbar.
;-]