Well, I actually made it through :smile49:
After replaying a few times, I managed to defeat the first three heroes, then finished off the fourth, gave the city to the Demon faction, and continued expanding westward. Slowly but surely, I eliminated them all - after the first ten battles, I hardly used manual combat anymore; the auto-battle decided everything ¯\_(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 in their starting location throughout the game.
But proper development is key. At the end of the mission, Railag was walking around with 500 mana and 30 spell power :smile44
And development is really important in this game. Bad and good development are practically two different games.
In short, the main thing was to survive the beginning