Okay, so I am having the worst luck with defeating Xaldin at Beast's Castle...

I recommend that before you battle Xaldin, have Donald in your party, so that he can heal you when you are low on health. Have your Cure spell in your shortcut menu as well.

In the beginning, you can attack Xaldin physically, but the main objective is to store up Jump attacks by using the Learn command. You can only do this when close to Xaldin, and when he's executing an attack. If you aren't able to avoid his attacks, just get hit by them, use the Learn command, then retreat when your HP is low and heal yourself. Then when your MP is back, return to stock up on more Jump attacks. Retreating is easier when you have mastered the Quick Run ability.
Once you have several Jump attacks, use them against Xaldin, then attack him normally when he's stunned.
Later on, Xaldin will use his lances to fly off the bridge. When he uses his beam attack, which is difficult to avoid, remember to watch your health and heal yourself when needed. Continue to use the Jump attack strategy on him and he'll go down.

You can use Valor form to attack Xaldin, but he counterattacks a lot, so use the square button to retaliate.

If you're still having trouble, level up. This will increase your Strength and Defense stats. Alternatively, you could use Strength and Defense Boosts. Load up your accessory slots with healing items, and learn Xaldin's moves so that you can use Guard at the right time.

You can also fight along the way but don't worry mickey can save you if you are defeated but keep in mind that mickey save all the time

Level up Sora until you feel he's at a good level, make sure you use your Forms, and make sure Sora has your strongest keyblade to use. Then you'll be able to beat him.