I used a different tactic.
First, I went to Lyudmila and took on her quest. After the gates opened, an Orcish hero ran up, and I defeated him. Then, I defeated another hero near the Castle, but I ran out of mana. I had to return to the Castle to gather more troops. In general, I defeated Sharka and returned to Lyudmila.
I left her to guard 3 Castles, which she did very well, and with Anastasia, I gathered the troops and went to the demon cave. In the cave, I managed to complete the quest for the staff, but I had to teleport to the city for reinforcements because the demons had already surrounded me and were trying to leave the cave.
Then, with a strong army, I finally went to eliminate the demons, and with Lyudmila, after buying her an army, I went further southwest to take 2 Castles. While Anastasia was dealing with the demons, Lyudmila already took 2 Castles and all the forts. The Necromancer tried to attack Lyudmila, but without success.
When only Miranda's Castle remained in the southwest, the Necromancer stopped attacking, and I sent Lyudmila to visit all the useful locations (like defenses/arenas, etc.). In general, after visiting all the necessary buildings with both heroes, they both reached the maximum level.

I found the mission not very difficult, but I did experience a bug: sometimes, something would glitch, and the crystals, as well as other resources, would not increase the next day. Restarting the game helped.