yep there are a few tricks about to make the most of certain maps. Duplicating foes are excellent at gaining, just whack the main villian down to very low health and each clone/duplicate they make starts with low health, easy killing for very high rewards. Also you can tag-team those experience/prestige canisters by getting all your heros near them, pausing the game, and having each one use the container, then unpausing. Everyone will get it at the same time, for multiple bonus instead of just having 1 character pick it up.

Also don't sleep on custom heros. They usually can be vasly superior to the in-game heros even though you can't level them up after. In-game heros have skills that aren't useful, or needed, and not great stats. you can create custom heros tailored to certain situations that are really much better than the story heros. Very easy to make much better melee characters or long range snipers than the in-game heros. You can have a great melee slugger who isn't slow, doesnt have useless skills, and still function vs melee or distance foes and move around well. Or you can make a distance attacker who can actually hit things.

Once again, if you are finding the game hard,change the difficultry level in the options screen when the game first loads.
It is odd that the manual for the game does not even mention the game has difficutly levels.

Custom Heroes have the problem of not being able to level up. Make a low cost one early, becomes not so useful mid or late game. Make an amazing late gamer, you have to wait before getting to use them.

In addition to the Blackbird description above, Iron Ox is slow as heck but solid in Melee and tough.
Supercollider is more late game but fast and tough, if you have not tried him do another run through.

In FFvsTR:
Bard is all about Crowd Control effects.