I don't quite understand what the difficulty of the mission is. Here's what I did: on day 6, I captured the castle with Duncan. Then I gave all the human troops to Frieda, and together with the dwarf, they captured all the mines in 1.5 weeks. Then Frieda and Ilay went to the Dungeon, and after 2 weeks, there were no more demons left (Frieda was doing the work). My losses were insignificant. Ilay captured the Dark Elf castle. When Frieda was leaving the Dungeon, a red hero somehow got locked in there and got thoroughly beaten. Frieda took a 2-week human reinforcement and wiped out 10,000 peasants in the garrison (losses were only 5 griffins). She defeated the red hero, gathered a 2-week reinforcement, went through 2 dwarf garrisons without significant losses, and defeated the dwarf king, losing barely 0.25 of her army. I completed it 2 weeks ahead of schedule.
P.S. Be sure to use training grounds.