You can really complete this game at level 30 (well, 30 at the start of the final dungeon, you gain some levels in there). If Celes is level 30 at the start of the WoR, everyone else will start at 30 or above too.

All you need is some good gear, and to prepare well for the bosses. Put Mog in the weakest party, and use the moogle charm to avoid enemies, then when you have to fight a boss with that party, unequip everything from the other parties and pimp them out.