You need to use the break/warp snipe reaction commands. When he returns to the
platform lock on to him, so you can keep up with his location. Use reflect magic
to hit his attacks back at him. This will daze him. When he is dazed run up to
him and attack him. Avoid black portals, and attacks when he is above you.
When the platform shrinks to a small square attack Xigbar in short bursts. Keep
moving to avoid attacks, or use reflect repeatedly to shield against them. When
shot are fired from every direction run circles to avoid them.
plz help me i have a preoblem i cant beat Xigbar in kh2 plz help anyone plz give me advice

Try Gamefaqs.
There should be a faq you could read for whatever tips you need for the game.
You need to use the break/warp snipe reaction commands. When he returns to the platform lock on to him, so you can keep up with his location. Use reflect magic to hit his attacks back at him. This will daze him. When he is dazed run up to him and attack him. Avoid black portals, and attacks when he is above you.
When the platform shrinks to a small square attack Xigbar in short bursts. Keep moving to avoid attacks, or use reflect repeatedly to shield against them. When shot are fired from every direction run circles to avoid them.
I hope this helps.
thanx