After replaying it once more, I managed to defeat the first three heroes, then finished off the fourth, gave the demon city away, and continued expanding westward. That's how I gradually eliminated them all - after the first ten battles, I almost didn't use manual combat; everything was decided by the autobattle ¯\_(ツ)_/¯
The demons behaved strangely - one ran to my city and captured it, after which it didn't leave at all, and the second and third simply stood still at their starting point throughout the game.
But the right build is key. At the end of the mission, my Railag was walking around with 500 mana and 30 spell power :smile44:
Added after 32 seconds
In short, the main thing was to survive the beginning.
Падение Грифона 136x136
Рельеф - 40%
Скрипты - 15%
Геймплей - 20%