Here's what I did :
Level up to LV50, and never enter into Valor, Wisdom or Master Form for a long time. Otherwise that'll add on to the "subtractors" and make you end up in Anti Form instead.
Have four Hi-Potions (autoreload) with you.
I usually have Sora, Donald and Goofy in my party (Donald can help me perform that Comet limit attack, which works really well).
Quick Run and High Jump should be at least at Level 2.
Then after that, the rest, as they say, is based on how Xaldin moves : evade at the right time, hit at the right time, and get the opportunity to learn up Xaldin's Jump command (preferably 9 times).
Oh, and use Reflect whenever he mounts that dragon-thing and attacks you with wind.