well this sucks

[The Man]

Strategy:
1) Walk around
2) Kill stuff
3) Heal if needed
4) Level up
5) Profit

You forgot the real step 5, which is ????. Shame on you.

On topic, try to keep a balanced collection of jobs for each of your characters. The quickest level up spot is a ruined temple in the second world that has some sort of undead dragon or something that gives you a crapload of experience points. I think I once actually levelled all my characters up to 99 there because I was supremely bored; in the process I mastered most of the jobs for most characters as well (Movers at the end of the game helped me finish those off).

If you're playing on an actual GBA, you won't have patience to level up to 99; had it not been for frame skip, I wouldn't have either. There really isn't any realistic reason you'd need to level up beyond about 50, unless you want to tackle Shinryū, Omega Weapon or the bonus areas.