So you don't like Summons? You still need a white mage if you're not overleveling, so you have two choices Eiko or Dagger. Eiko has better options for White magic though. I chose Dagger cause I like summons in a midlevel game and they're easy to get...

Zidane has to be there. If you steal successfully once in every random encounter the entire game, his thievery skill will be powerful without having to level him up (I did this and had approx 4000 damage by the end of the game equiping all Speed + items I had).

I like using Vivi, because Flare is a powerful spell and it can be acquired faily early and easily compared to most other high damage equipment.

Steiner is my last Character. His magic sword skill allows him to do big damage with Vivi in the party (It's like two flares a round). While he does insane damage if you do all the side quests (get shock and climhazzard), he does decent damage without and he is fairly cheap to equip.

Use these guys, and get Auto-Regen on all of them as well as Auto-Potion and Chemist for those who can learn it (Dagger, Steiner out of this group). You should be able to learn these throughout the game. About halfway through the game sell all your potions and auto-potion will use high potions on you (everytime you get hit, 450 damage recovered [900 if chemist is equiped]). Spend the of your ability points on any status-nulling abilities or something like autohaste and you have a powerful party without too much effort (or at least one that is very hard to kill).

Oh yeah, make sure you get Level-up and Ability-up for everyone so you learn spells and abilities quicker and level up with less fighting.

The most important piece of equipment for you to get in this party is the Black Robe for "Flare" spell. Get it in Disk 3 at the synth shop in Dagurrero (spelling is probably off).

The rest is optional: Just keep buying Steiners and Zidanes most powerful weapons and if you have extra cash, go to the auction house and buy up key items till you get "Magical Fingertips" give them to a guy in Dagurrero (spelling is probably off) to get Excalibur for Steiner. Buy a copy of any weapons or armor for the white mage and Vivi to get them their spells. The game doesn't require you to level up at all, and I had little to no trouble with the last boss following these guidlines.