Rexsar
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Hmm... No one really attacked my Succubuses, so I ended up with a lot of them. All the aggression was directed at Imps and Cerberuses. But gating helped. I blocked all paths for the enemies, disabled ranged units with Seduction and Blindness. By that point, the summoned troops were ready to attack. But at that stage, the main role was played by the offspring; everyone focused on them.
Added 2 minutes later
Don't rush to capture the garrison next to Thor Hall. It's better to build up your army. But then the AI's strength will decrease significantly. If you defeat Hangwul, it will be a good foundation for victory. Then you can start forming a mixed army. I only hired Priests and Jarls among the Dwarves.
The garrison in the 2nd Castle has 95 Jarls. So, at the moment, the AI has 3 Castles.
The Imps that I had were used to reinforce the Elemental army + the 1st garrison on the way to the 2nd Castle. Now I have recruited more Imps, but I encountered a hero.
So, I lost Imps in these 2 battles. Then I broke through the upper garrison (on Utopia), took Utopia, and cleared out minor mobs until I reached the garrison in the 2nd city.
You can defeat the Elemental army without losses, in principle. But it won't help much.