There are three types of heroes, each of which is strong against a specific type of enemy. Enemies are marked with an icon above their heads that indicate which type of hero they are weak against. Quickly switch heroes by knowing which is best to handle the enemy at end.

Collect Mana Points from the blue orbs dropped by monsters to access your heroes' power-ups
Switch your heroes when possible to level them up evenly. Each hero should be kept balanced with the others to handle any situation.
Unless absolutely needed, do not get help from other heroes. If you do, your own heroes will lose experience because any enemies killed by the guest hero give you no experience points.