Ulitma Weapon, Eden, and Omega Weapon

WTF is up with this boss fight? I'm trying to get Eden for the first time and I've been trying to beat Ultima for the past couple days. This boss is ridiculous! I try to start off with summoning Cerberus like I usually do for hard bosses and he casts Light Pillar killing Cerberus instantly like he knew what I was trying to do. This boss gets multiple turns too like Ozma where he might do three physical attacks in a row and then Quake. I only managed to survive for about twenty minutes one time where for some reason he didn't move as fast and mainly used Quake and Meteor. I was able to set up with Cerberus and buffs. Regen is weak in this game. It's not super abusive like in IX.

Oooh, what should I do? I want Eden and wanted to go for Omega who I never beat either, but he's even harder.

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.

The party I was using was Squall Level 100 with 9999 HP, Shiva, Diablos, Bahamut, Rinoa Level 52, and Irvine Level 53.

What I usually do is try to get Rinoa to cast Cerberus immediately, but a few times Ultima will just go for light pillar killing Cerber, I would draw with Squall, have Irvine cast Regen by Draw. If I can get Cerberus then I'm triple casting Hate, Protect, and Shell. I don't have auto-haste yet. I assumed the the command abilities suck so I never used them. I've always used Phoenix Downs and Life spells for reviving. I tried my hardest to go for Invincible Moon, but he either kills Rinoa before I can or she'll do Angelo Cannon instead.

I might try that Selphie strategy. It's cheese, but it'll get me the win.

Simply leveling your other characters a bit could probably do you some good (though you might have to leave the area for a bit to do so properly). Also, making sure you Junction Earth to your elemental resistance helps a lot, especially if you can get the Quake damage reduced to 0 (or even better, absorb it).

If your party can survive a Quake and/or Meteor after Gravija, you can spam Limit Breaks rather than focus on healing up whenever Gravija hits your party. Also, make sure you hit Ultima Weapon with the Meltdown spell (and that you get the Def 0 effect from it) at the very start of the battle. Ultima Weapon has high offensive power and very high speed, but can also go down pretty quickly if you get some powerful Limit Breaks off after a Meltdown.