Or you can master the X-Magic atribute off of the Red Mage and then use that ability on any of the other classes, as you have one ability slot free to assign. Or you can change your character's class abck to normal and you have two (or three, I can't remember) slots to assign abilities to. You can have a Black Mage/Wizard and then assign the X-Magic to their second ability slot and cast two of any Black Magic spell that you can.