What team are you using besides Squall? Don't worry about HP and defense because you can mitigate Quake with elemental defense junctions and Light Pillar is a 100% KO move that I think Defend can work against, but since he'll use it randomly, it's not very helpful like in the Omega fight. Speed should be the priority with good junctions to the speed stat, auto-haste if you have it, or getting haste on the party ASAP. You may want to use the Revive skill or phoenix downs to revive party members back with low health so they can automatically use limit breaks.

You'll want to start the fight with using Doomtrain or Meltdown to give him Vit 0 so all attacks do max damage. Make sure Squall has Lionheart to do max damage and thankfully, his Renzokuken move does seven hits for this fight, which with the Vit 0 status and using Lionheart with each hit doing max damage, you can effectively kill Ultima with just that move. Even at max level, Ultima's health caps at 160,000 and 24 hits of 9999hp equals to close to 240,000 hp of damage. Anther strategy is Zell's Armageddon Fist which is a bit boring but will kill him in one limit break if done properly. Alternately, you can use Rinoa and either hope she pulls off Invincible Moon to make the party immune to all damage and take your time, or you can configure her magic to only have the Meteor attack spell (no other attack spells, including status magic) with a good Junction to her magic and auto-haste, use her Angel Wing limit and her Meteor spells will do max damage as well with Vit 0 and can kill him with three castings. Another strategy is to set the battle mode to Wait and use Selphie's Slots to get The End and just finish the fight like that. Irvine with Fast Ammo and a good speed on the R1 trigger could also take this guy down pretty quickly with Vit 0.

Omega is actually a bit easier to deal with due to his moveset being stuck in a pattern, so knowing the pattern will help mitigate the problems with him. His pattern is basically:

Opening - Lv. 5 Death
1st Round - Meteor
2nd - Megiddo Flame (does 9998 point of damage to whole party, your character must have full health not to be killed by this)
3rd - Gravija (75% reduction of health, can't kill you.)
4th - Terra Break (sixteen random physical attack that do about 4000k a hit. Have the party use the defend command to survive with no damage)
5th - Ultima
6th - Light Pillar (max damage attack, just revive whoever it kills.
7th-end - restarts from Meteor and cycles through again until killed.

As long as you know this, you can defend against most of its attacks and frankly meteor and ultima are hardly the game ending spells they were in VI and VII so don't worry about them too much. His health maxes at around 1,200,000hp at Lv. 100. If you're playing the PlayStation version, he's always this level, if you're playing the PC version, he matches your party level and can be easily beaten with the right junction. Most of the character strategies I mentioned above against Ultima work here as well, though I will point out that Omega absorbs all elements, so make sure not to have any elemental weapon junctions equipped and only use non-elemental spells like Meteor and Flare.