If there's a character or two you absolutely hate, you can get away with not leveling up those two. You get 14 characters if you don't screw anything up, and the most you use at one time is 12.

I find that to be one of the most fun aspects of the game-- keeping my party as level as possible. I always take my weakest characters so they can get the experience, then when one of them surpasses the people on the sidelines, swap that character out for a weaker one. That makes you use lots of different party combinations and you never have the same party for the same task in replays, except of course for where your party is determined for you.