A few thoughts:
Units. As you can see, most factions have the same units. For example: Basic - 1 unit with physical attack, 1 ranged unit, 1 support unit (magical attack or attack and return). Elite - 1 elemental unit, 1 unit with physical attack, and 1 unit with magical attack (mage). Champion - there is a champion. Therefore:
Basic: Minotaur, Beholder, Beast.
Elite: Witch, Manticore, Shadow (elemental).
Champion: Faceless One.
But I feel sorry for the Hydra....
Their main specialty is, of course, increasing magical attack.