Part of what makes the formula of dungeon crawlers is that enemies are spaced out only a few to a room. In this game, I can order all of my troops to retreat a few rooms away from the hero, then swarm the hero all at once.
I have no idea how you could balance this unless there was more than one hero and you were trying to keep them all away from a central treasure room.
At that point, you could add elemental damage, so there has to be some strategy of dispatching the right monster depending on the hero's weakness or figuring out how to divide up your army when fighting on 2 fronts.