Each hero has an elemental affinity which will affect the damage he/she can inflict and take, as well as the bonuses they can get from the party leader and a helper. As a summary, you deal 30% more damage to a target with an element you're strong against, and you'll also receive 30% damage from that target. You can refer to the image below:
Important Note: Light and Dark elements only deal bonus damage in battles outside the Arena.
As a general rule for dungeons and adventure maps, always bring a party that has elemental advantages against the enemies in that particular map/dungeon or at least don’t include anyone that’s weak against the element of the enemies in a particular dungeon. You can check the dungeon’s Region Data before entering the battle.