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An alchemist almost completely erases the need for a white mage (while still doing some nice damage), and black mages aren't nearly as impressive as in FFX, so unless you really like magic, you can forget them (a gun mage will be just as effective for most fiends). A Dark Knight's Darkness ability, along with an acessory that increases HP, is a strong move, hitting all targets. Trainer, although gained late in the game, is also very effective; working as a sort of 'light' version of each character's special dressphere.

Have them complete their startout dresspheres first, then focus on berserker, dark Knight, and the two types of mages

I completely agree with that

I have found that i like the Dark Knight and The Mascots dress spheres. When you use dark knight, you can use black magic and they have their own special attack ( it recoils a bit though ), plus they use weapons too. The Mascots attack is high in damage. I must say though Dark Knight is my favorite. be sure to keep a white mage in your party though.