Regarding the tactic of using Demons against slow melee units: In campaigns, I do the following: I surround my creatures with summoned units. When neutral units approach, my summoned units are already out of their reach. While they are fighting the summoned units, ballistas and the hero deal the main damage, and then the main troops finish them off (this works best against large creatures, as you can block them with fewer units, for example, you can block Ents in the middle of the field).