how to defeat this monster?its so hard to defeat him..

Hint: Meltdown.

If you've gotten that far, just keep using Renzokuken and make sure you have the Lion Heart gunblade. If you fought Behemoths on the way down you probably have the Barrier item to teach Quistis Mighty Guard! In this game it does the usual wall and sometimes casts Regen, Haste, and even Aura!
And for Healing purposes have Selphie be your other member and use Full Cure on her limit, it comes up a LOT. Forget White Wind and Magic (you need magic junctioned). Quistis also has some offensive Blue Magic that may work as well...

So yeah, just have Squall be your tank, and Quistis and Selphie supporting you the way I mentioned. BUT make sure you have a lot of Life/Full Life/Phoenix Down or Alexander's Revive, because the Light Pillar is impossible to survive with walls, you have to be in Invincible status, but you should be fine with just reviving.

Also if you want, teach Quistis Shockwave Pulsar! It breaks the 9999 limit, and it really does not take as long as it sounds, sure you need 100 Curse Spikes but I mean, not only do the Tri Face monsters in the area on the way down hold them, but you get as many 6 or 8 at a time when you Mug them or defeat them. And they're very common so netting 100 should be no problem. This way Quistis can be a tank as well.

Oh, and don't forget to draw Eden, can't miss that.

Once again, besides the drawing, before you do anything make sure Meltdown hits. It doesn't always work, he has to turn a funny shade of purple and then you'll know it worked.

It's also a good idea to make sure you have Phoenix. I'm pretty sure it had to save me once through this battle.

Simple (you can beat Omega this way too). Have Selphie in your party with very low HP. Have her equipped with Initiative so she can cast a Hero or Holy War on herself. When her ATB gauge is filled, have someone else cast a spell or summon a GF. As soon as they start casting, open the cover or eject the disc. This causes the game to freeze, but you can still select which action you would like to take with Selphie or anyone else who's ATB is filled. Use Selphie's Limit Break and keep selecting Do Over until The End is selectable. After you select that, close the lid or disc tray (the game will run kinda slow after this, but will get back to normal speed in 10-15 seconds). After the character is done casting the spell or summoning a GF, Selphie will cast The End and there you have it. Ultima is no more.

Make sure to draw Eden before hand.

I took on Ultima for the 1st time when I was 16. I couldn't beat him for two days. He made me cry.

Heros/Holy wars are good. Rinoa's "invincible moon" is a lifesaver, as is "wishing star". And Squall's Lion Heart. Basically, don't even think of fighting any of the optional bosses without Lion Heart. (as stated above, Meltdown is crucial).

After reading a lot of these strategies/warnings/whatever, I thought I was gonna get a Game Over within 20 seconds. Well, it was a Game over, but to Ultima. I fail to see why he would be so difficult...

Let's see what happens when I get to Omega ^^