Certain elements are stronger against others. See below which element is best against which:
Water deals extra damage to Fire
Fire deals extra damage to Earth
Earth deals extra damage to Water
Dark deals extra damage to Light
Light deals extra damage to Dark
You can always remind yourself as to the hierarchy of the elements by looking at your Draw Card button while in a Dungeon.
It is also beneficial to plan ahead before entering a Dungeon. Let's say you're in Chapter 2 and want to conquer the Spruce Forest. When you tap on this Dungeon, you see a list of enemies. This dungeon has a lot of Earth enemies, so you want to make sure your party includes a Fire Hero because it's strong against Earth.
But also remember that having several different elements in your party is important, because the mana cards have five different elements that you need to activate your heroes.