No, my tactic was as follows: send a well-leveled Knight with powerful troops from captured castles to Grool, and even give him over 100 Archangels, but only with one goal – to kill all the mana-draining Imps. After that, it's just a matter of technique (speeding up, subduing, etc.). But the thing is, on the same day, a second, Koch-demon, showed up. Here, it seems, you need to kill one and then quickly retreat. I haven't managed to do that yet.

Added 21 minutes later
Yes, by the way, the second nasty creature actively uses dark magic, so without Paladins to dispel it, there's nothing to do there. I'm feeling a bit lazy, but good luck to you, heroes.