I usually use Firion as a mage, both white and black. Maria, until my latest playthrough, is a straight up monk. However, last time I played I gave her a shield in one hand and left the other barefisted until I got a Blood Sword, and she became almost unhittable. She was blocking nearly everything. Gus gets to be my warrior and backup mage. I try to avoid making Gus a mage because if there are only three party members, I can't go back if I hit a wrong button while targeting. I try to equip Firion for spirit; Maria for agility, speed, and protection from confusion; and Gus gets the best of what's left. BTW, I play the Origins version. I think it takes ~50 castings to level up a spell.