Very nice and addicting concept but it's too hard. You can collect group of small skeletons, but than there is too much big groups or tough enemies are spawning. Horde in the middle is getting bigger and bigger, and even if you managed to collect some tough guys or/and mage you can't find group small/weak enough to join your army without aggro of the horde.
Also you can't spawn one killed enemy, only a full group, so it's adding even more to the difficulty