Ice and Fire

Heh. I posted recordings of battles with him. He always used Blind on me, plus other minor spells. Basically, I managed to drain his mana using spells. I had a lot of mana.

Would Subjugation help? The last artifact of the dwarves isn't too far from Thor Hall. If you try, you might be able to grab it, if your hero has the remaining 3 uses. Or, if you have a save before the garrison is captured, you could try to build up an army and capture everything you need.

Subjugation would probably help.
You saw the screenshot I posted. By the time he reached the dwarven castle from the garrison, the number of Jarls had increased to 152, and the same happened with the other troops – there were 1600 harpooners. The succubi only subjugate these Jarls by 0.01 turns =)). Letting them cast – that's a stack lost immediately (minus the succubi – they attack them). In principle, if you get an artifact that reduces defense, they might wait a turn – subjugation and this magic will then affect the AI. Then just use them to attack everything that moves, while simultaneously casting Dispel on the archers (not a mass dispel, so the Jarls get a response). They'll be killed – move on to the next stack.
With this setup, yes, it's viable.
By the way, can you get Subjugation in Utopia? I farmed it for the second time – it gave me some good artifacts + 3 spells (Shock, Holy Word, and some other one).