Rexsar
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Well, that's true. At least Sigrif's specialization is working in all senses. That's a good thing. Maybe fewer Imps could have been deployed; they don't seem to be very effective anyway. They didn't steal much mana; they were defeated after the first attack by the Jarls.
The Jarls weren't bought for less.
They drain mana well. If you don't drain mana from the hero, he starts throwing Meteor Showers and casting Stone Skin, which is quite effective.
Added 1 minute ago
Yeah, it would be better to lure him into the Dwarven Castle. And use a secondary hero to bring troops from the caravan. Then proceed with the same scheme. Except that Hangwul is a good Dark Mage.
Magic doesn't matter, unless it's Berserk with Subjugation.
+ I have a skill that makes him spend twice as much mana on Dark Magic + add the Imps to that, and it will cast quickly.