Blue Mages are amazingly versatile if you're patient enough to build them, they learn the attacks of monsters when hit by them if the "Learning" support tech is equipped. These techniques are incredibly varied, and allow Blue Mages to deal decent damage and inflict status effects at the same time.
For an offensive spellcaster I would recommend an Illusionist, their spells attack all enemies on screen regardless of distance. I would recommend equipping him with as many magic rings as possible, putting him in a corner, then putting a Paladin next to him. This will allow him to repeatedly screen nuke the entire enemy team whilst being guarded and out of danger. They wield the typical Mage equipment: rods, staves, and robes. They also have low movement, but that doesn't really matter, what with their screen nuking, and all.
For a support class the best IMO would be a Paladin that you've given lots of White Magic, this will allow him to do his job (Be a tank with defensive abilities) even better than a vanilla Paladin. They can wield rather strong swords and can equip heavy armor, making them decent at attacking if things come to that, but their base movement is as good as a Mage's (It sucks).
I would recommend Ninja for your main offense, their "Double Sword" support tech allows them to wield two weapons at once, and also allows them to attack twice. That along with their Jutsu (Which allows them to deal elemental damage and inflict status ailments at the same time from medium range), makes them decent in almost all situations. Their throw ability allows you to toss unused weapons at your foes from long range, they also have decent base movement which helps them close the distance on archers and the like. Ninja should have Thief abilities as their second tech choice, as their high base movement allows them to run like a French Border Guard with cleats and a coupon for cigarettes. Also, stealing doesn't cost MP and Jutsu does, meaning they won't put the Ninja's MP on double duty.