I could manage well with only a couple of classes. In the beginning, I used Gunner, Thief and Warrior. If there was a hard battle against a tougher fiend, I usually changed someone into Songstress to cast Darkness Dance and had the other characters attack as Warriors.

Later I used Gunner, Gun Mage and Warrior, but I think a group of Gunner, White Mage and Warrior would have given me better results. I was using Heart Reborn to give me some healing spells and I had a lot of healing items, so I didn't have much problems, though. As for black magic, I hardly ever used it.

When I got Dark Knight and Alchemist dresspheres, I changed to them very quickly. My final team was Gunner, Alchemist and Dark Knight. Trigger Happy, Darkness and Stash Mega-Potions were everything that the party needed, really, and Dark Knight was nicely immune to most status ailments.