The only things in the post game where you definitely need a specialized party are the legacy bosses. Otherwise, you're probably good with a balanced group, provided you're a decently high level and you've been loading up on skills/skill points. Definitely get a sage and max them out as soon as you can.
I have a feeling they're not really the type of party you have in mind, but here are the two setups I used most in the post game:
3 rangers/priest - Rangers are probably the best all-around class, with decent attack, Zing, and the highest deftness in the game, which is good for stealing. The only thing they can't do is Multiheal, so I add a priest.
3 gladiators/priest - When you just need to kill everything as fast as possible, go with gladiators. This works well for legacy bosses up to the 30s-40s.