The key is to keep your levels low, while still gaining a decent amount of AP. Then you just obtain items through Triple Triad -> Card Mod (Quezacotl ability). By this you can get high-level magic like Tornado Firaga, Haste, Regen, Curaga and such at disc 1. Actually, Curaga is the easiest of them all, just get Siren to learn recovery magic refinement and buy 30 tents, then you've got 300 Curagas for your HP right there (about 2-3000 HP). Seeing as the enemies are about the same level as you, you'd find that beating a level 20 boss while you've got 4000 HP and 80 strenght is pretty easy.