Ice and Fire

I always cast Seduction/Subjugation on the enemy hero, and from the second turn, I try to eliminate them. Or Blindness. In any case, you need to "disable" the archers before they get a chance to attack. But this only makes sense if you have a decent army.

Added 1 minute ago
I'm thinking, does the AI always eliminate Imps first? What if I put a small stack of Imps in front, and keep the rest in reserve? Will the AI spend its spells on them, or will it be smarter?

I have 140 Succubi and 340 Imps. The AI has a hundred Djinns. The garrison has 95 units that need to be defeated.

The hero eliminates all the Succubi in the first turn. If you put fewer of them, it eliminates the Imps (there are 270 of them).
The garrison eliminates either the horned stack or places a stack of Imps – it depends on luck. But neither option is ideal.

So, in a battle with the AI hero, you lose at least all the Imps + Succubi – and that's without counting the other losses.
The garrison eliminates at least one more full stack of Imps or horned creatures, and considering the losses in the battle with the hero, even more.
So, after eliminating the hero and the garrison, there is no army left. Eliminating the hero without the garrison also doesn't make much sense – it will quickly create a new army. It merges its troops into one. The other heroes run around with single units.