However, there seems to be some kind of bug with the Necromancers – or rather, not a bug: for some reason, they noticed and actively destroyed the Secret Crystals, even though I placed them correctly; I don't know why that happened. But you can also win with brute force – which is what I eventually did.

Heroist, thank you! Yes, I completely forgot about the Secret Crystals! They are very helpful! Even when enemies destroy them, they take damage. And if the "Varenik" moves quickly, he can re-place the crystals in time.

Added 2 minutes ago

I think the most difficult battles were against the Guardian before the nerf – without Dark magic, it was impossible; he had enormous resistance.

I played the first version, and in that version, this monster simply removed Dark spells from his units. That's why I replayed it 9 times. In the end, Magic Resistance worked, and he couldn't dispel Subjugation. Otherwise, I would have lost. :)