Ice and Fire

Good luck with the secondary hero! Later, it's better to gather troops and save before capturing the garrison, just in case... ))

First, you need to kill the hero with minimal losses.)))
I have 20 devils, 49 cave goblins, 88 horses, etc.
The computer has 70 dragons, 105 jarls (with the Flame Wave skill), etc.
After that, take a garrison with 2 castles, each with 95 jarls (with the Chain Lightning skill), a legion of guardians, etc., in the same vein.
And then capture the castle, where Brand runs around with a group of dragons and a horde of guardians))

Gathering troops is not an option. The computer accumulated 70 dragons in less than 2 months, not counting Brand and who knows who else. You can't outdo that.