if you are level 50 when you beat gafgarion, you're doing fine. With all your fighter units master all of the fighting classes. With your magic units master all the magic classes. make a few people do a little of both to get the harder classes like Mime, bard and Dancer.

Experiment with different combinations of classes and skills, soon you'll find which abilities are the best and which weren't so good. Make an elite team. Or have fun and challenge yourself by making different teams (all White Mages, All Calculators, All Monsters etc.)