OK, 2 things:
1. Maitaj, I would say Sephroth is the HARDEST boss in the game, 10 times harder than Xaldin.
2. I can help you with defeating Xaldin. Here's how to beat him:
You should be at least level 35 for this battle. Be equipped with Hi-Potions, ethers, and all kind of medicines.
Now, in the beginnig of the battle, Xaldin will surround himself with his wepeons, so DO NOT go next to him, don't attack him at all. I would suggest you get Goofy to the party, and use Knocksmash immediately. Now Xaldin will begin to start slashing you with his lances, so block them with guard. Later on, Xaldin will jump into the air, and slash down towards you. Here's the important part. When he is about to slash down, jump into he air when he lands, and the Reaction Command 'Learn' will appear Press triangle as much as you can. Your attack command will turn into 'Jump' or 'Jump x(number of times you get learn)'. Immediately begin attacking with Jump. This will unleash Sora into the air, and slash down onto Xaldin. This infilicts quite a lot of damage, and gives you a chance to dodge his attacks. Later on, Xaldin gets serious, and begins glowing green, and this is where you have to RUN AWAY, since he will unleash a lot of attacks, that do a lot of damage. Then, Xaldin will ride his lances, and they will form a dragon on the side of the battle area. Preferably, do not stay on the side, since this attack may hit you twice, so stay in the middle. Now, when the tornado comes towards you, Reflect immediatly, so you can protect yourself. There is a way you can dodge this tornado, by using High Jump level 3 obtained from Valor Form. Anyway, there will be times where Xaldin will repeat his moves, so just do the same things I told you and you should be safe. If you want to do some damage, I suggest summoning Genie, and turn him into Valor, since Rave does quite a lot of damage.
If you die, do not start panicing, since there will be the possibility that King Mickey may save you, so just use the Pearl attack when you are using him.
Phew, this took me long to type
Anyway, good luck in defeating him and completing the game.