It wouldn't be strange if that commander awakened as a devil himself, alone or aided by one of the top devils of the empire. He seems to be stretched to the limit and is full of hatred, greed, stress, envy, and other splendidly demonic emotions. He also seems to be taking Shin and Cecily's titles granted by the pope almost like a personal insult.
I reckon one big mistake the human army is making is to think the devils are some unified force. It should be obvious that's not the case even without having seen the scenes among them that we have seen. People who became devils because of being oppressed by the nobles aren't the first ones to volunteer to serve under someone, quite the contrary. The devils ought to be naturally chaotic, so why would they sit waiting in a single fortress or look for a nice, big, organised battle?