It works on most of her battle.
Then some whoring of "hero’s trial" and auto-life
My best team was miz trains; squall; and zell
If Selphie is in your party, her rate of getting The End (which will instantly kill any single form of Ultimecia) is significantly increased when her (Selphie’s) HP is closer to 0.
You can also use Heros, Holy Wars, or Hero-Trials or Holy War-Trials. To get these, you can use Card Mod to convert the Laguna card to 100 Heros, or the Gilgamesh card to 10 Holy Wars. Also, 100 Curse Spikes (dropped or mugged from Forbiddens in Centra Ruins) can be converted into 1 Shaman Stone, and 10 Shaman Stones can be converted into 1 Hero-Trial, etc….but it may be easier to use Rinoa’s Angelo limit break and hope to get Invisible Moon, which serves the same function as a Holy War.
One last tip might be useful: If you’re worried about dying, use a Phoenix Pinion in a battle before the fight with Ultimecia (50 Phoenix Downs = 1 Mega Phoenix, 3 Mega Phoenixes = 1 Phoenix Pinion). That way, if all characters simultaneously in any later battle, there’s a good chance that the GF Phoenix will attack the enemy and revive your entire party, bringing them to 1/4 max HP. That’s all I think you need to know. Good luck!
Or…for a less-stressful way to defeat her,
You could keep your levels low by carding. Then you could junction some crazy-ass powerful magic and hand her ass back to her as if she was a level 7 fighting three of your guys with level 100 stats.
Get a lot of Aura, and have a team with Selphie and Quistis (or keep killing everyone until you get them), make sure Quistis knows Mighty Guard, White Wind, and Shockwave Pulsar is also pretty useful. Rely on Selphie for healing by using Full Cure which comes up a LOT and is free and doesn’t run out. Have Squall keep using Renzokuken. It’s good if you have the Lion Heart so you can get that attack. Or to save Aura, Quistis’s Mighty Guard sometimes gives Aura status. Also you can rely on Selphie for stuff other than Full Cure, like Wall, Meteor, Ultima… or The End if your lucky.
And if you have Shockwave Pulsar use it every opportunity Quistis isn’t doing something else.
But anyway, it really all comes down to your level. Make sure your characters level up good. Fight monsters in the two islands called "Island Closest to Hell/Heaven. Use the non-encounter ability from GF Diablos if you have it and walk around the edge of these two islands. The islands itself are surrounded with hidden magic draw points. And those are the powerful spells like Meteor and Ultima. Keep walking around until you get 100 of each magic you want.
Junction good magic to improve your stats. Preferably, junction the strongest magic you have to Squall’s strength. When you fight Ultimecia, keep Squall in a low HP or use Aura magic or Hero Drink to activate his Limit Break. Hopefully, you have his final weapon as that will trigger Squall’s strongest limit break which deals a hell lot of damage, especially if Squall’s strength is badass. Other characters can play support to Squall, while maybe doing some damage here and there if they are strong enough.
Other than that, if you don’t feel like leveling up, then while you’re in battle, activate Selphie’s limit break. When you are scrolling through the spells available, pop open your PS2/PSX disc tray. The game will stay still but you’ll still be able to go through Selphie’s limit break. Once you find "The End" close the disc tray again and activate "The End".
Sitting on top of 27 victories over Ultimecia without dying once, I’m happy to say it works