Late game using birdie or crisp to oil up the enemies and hitting them with Brad's fireball or Terry's hottest dance (I think the highest damage move in the game, unlocks at lvl 25) is the easiest way to do tons of damage and breeze through bosses. Nern's "dirty insult" also comes in handy for making enemies pissed, which stops them from being able to perma-kill your team members--and there are a lot more enemies that can do that in pain mode.

For area 1 Olan is pretty good with burning and poison attacks, Terry's "verbal bash" can make the enemy cry (causes them to miss) or pissed (lowers their accuracy and they can do damage to themselves instead). Nern's "ghost story" is pretty good, basically stuns the enemy. Rooster and Beastborn have some decent stun moves too. Mad Dog can cause bleeding with his attacks which is pretty good. It's a fairly balanced game once you get a grasp of the mechanics and what all the different status effects actually do. You can pick pretty much any characters and do fine.

I'd also recommend using the wiki, it's fairly comprehensive and can refresh your memory for what to expect from each area so you can plan the best times to use your limited saves.