Save before a dungeon or anywhere. But most importantly dungeons. Then go in and explore and get all the items. Then leave, sleep at an Inn, go back, save outside and go all the way through as fast as possible, ignoring all fights, in order to get to the boss.

That's what I did. Meeting a boss with post-exploration health and MP is not good.

Incidentally, I found a neat way to level up jobs super fast. Just have everyone guard (or steal and defend) for about 8 rounds, occasionally healing when needed, and then kill the last monster.