Yes, don't waste the Gem Box. As another note, using the Moogle Charm makes the Fanatics' Tower a joke. (Well, any dungeon a joke )

Have Life 3, but you can also Osmose or Rasp Magi Master so he doesn't use Ultima. But that takes a long time, slaughtering Cactuars for Life 3 is easier.

And, equip the Gem Box and Economizer to your best spellcaster!