These made the game very easy for me:
- the Alchemist and Dark Knight dress spheres (forgot where they are, but they're not too hard to find)
- Cat Nip (equip it on someone who knows Trigger Happy lvl 3 and make it so they're hp is yellow)
- accessories that increase your HP/Strength/Defense (obviously )

Early in the game the Songstress dress sphere helped a lot