For ruby dragons before Bahamut, just equip initiative and then use lvl down from tonberry king. Then breath should only do 1000 damage to all. Keep lvl them down and the battles easy.

Ultima weapon is really easy. Just get Aura from islands closest to heaven and hell (with enc none) and use it on Squall. Lion heart after a few tries should come up, killing it in one go.

For malboros get Quistis on really low health with degenerator + initiative and kill it before it uses bad breath.

For Odin, 1st time you should work out what you have to do to get to him. 2nd time arrive really quickly, get his health low with aid of scan and then draw triple like crazy, before killing him in time.

And in the disk 3 spoiler Seifer fight take time to draw Aura and mug a holy war.