I make Maria the only spellcaster and keep her in the back row with no equipment (except the black robe and ribbon later on) , because the heavier the equipment the less powerful or successful a spell is. I only use the following spells: Cure, Fire, Haste, Berserk, Toad, Exit and Life. Once you get Toad you'll rarely over use Fire again, as Toad is the most powerful spell in the game. At a high enough level it can take out almost every enemy in one shot. For bosses she's gonna just heal and spam berserk.