This is what I always worry about with parties, because I like everyone to be at Level One Million before fighting advanced bosses.
I touched on this in the thread I started but I'll elaborate here: in FFVIII when you're faced with splitting up the party to head to the Galbadia Missible Base and heading to Balamb Garden to warn the headmaster and save everyone, etc. I liked how urgent the whole situation was and you really had to think about who could be useful in each party instead of sticking all the crappy people in one party and all the star players in another.