I always struggled with leveling early on, rather than later. I would find that I'd stray too far off and end up in battles against enemies that I really shouldn't be running into!
But I agree with you, killing off atleast two of your characters so that the other two can level and vice versa is probably the best tactic there is.
It's a shame that some of the more recent Final Fantasies don't have that same amount of hard work in the leveling system.