Fighting monsters is the main method of leveling your hero and captains. Monsters are divided into factions:

Not only do monsters differ by their faction, but they also have various traits, just like your troops! Any monster in the game has at least one of the following traits:

Certain monsters might have several traits at once, like a Storm Crow, which is both flying and a beast. Though there are no monsters in the game whose traits would match both strength bonuses of a regular troop. This is one reason why you should combine various units while composing your army when attacking high-level monster squads.