What I do is make Firion and Maria white mages/warriors (level up white magic and equip them with swords) and focus on Guy as a primary attacker (level up axe skills). I also give Firion and Maria a few black magic spells, but do not focus primarily on them. I use the fourth slot character as what they are intended to be (Minwu as white mage, Josef as Monk, etc.) For all, I try to level up HP the most.