I suggest refining Ultima from Dark Matter. Go to the Deep Sea Research Center and fight the fixed encounters of Tri-Faces. Mug/win Curse Spikes from them, and refine 100 of those into 1 Dark Matter (Siren needs to be level 100 to do this), which refine into 100 Ultimas each. Each Tri-Face battle should yield around 6-8 Curse Spikes, so the process won't take very long. It's much safer than trying to draw them all from Ultima Weapon.

Draw your supply of Full-Life from Bahamut and draw Meteor from Ruby Dragons when you have one party member killed. Definitely the easiest methods for both. Full-Life can also be drawn from Tonberry King if you're at a high enough level.

You can only draw Aura from the fight with Seifer in Lunatic Pandora, and that's pretty late in the game and a one way trip into time compression. There's a better way. Go to Esthar with Tonberry's Familiar ability and buy a bunch of Hypno Crowns and/or Power Wrists. Each of those will refine into 10 Aura Stones which can be refined into 1 Aura each.