Apach876
Hero.
The key to completing this mission is developing the economy with the fastest possible transition to training horses and angels, preferably by the third week. To do this, you definitely need to level up at least two heroes and clear the map in different directions. It is also worth making sorties and capturing dwellings as early as possible - this significantly weakens the enemy armies. Intercepting garrisons is quite possible, but not immediately; it is best to do this after killing the first demon and the appearance of Arantir, who can be permanently sent to the yellow player's territory and simply hidden in sanctuaries when enemies appear. It is also worth constantly buying peasants to get more gold, and after level 30 of the main hero, be sure to mentor Isabel so that horses train very cheaply. But, of course, success seriously depends on the starting layout - what stacks are guarding, what skills drop, how much of the army was preserved in the first battle. Therefore, I always role-play the leveling up in such maps. I didn't think there could be any problems here, as the single-player version, where the demon armies are 4-5 times larger, is quite (almost) comfortably completed. And I wouldn't play on Eve, the special ability of Zehir is definitely from there, and its development is very important + very unbalanced neutral forces can appear in place of random stacks.
Added 4 minutes ago
I can only confirm what Gerter and MasteR wrote. Zehir, as a "hero," eliminates all neutrals at the start, with very few exceptions. By the first week, both gremlin dwellings and both upgraded forts should be cleared. Further movement depends on the player's wishes. I usually captured the gold mine by the end of the week; money is very needed for the knight's castle. The basis of such development is Zehir's special ability; the destroyed stack turns into an elemental, which is noticeably stronger than the killed gremlin. Zehir easily gets fire warriors, around level 4-5. Getting an education is also not a problem if you managed to trade for logistics; he clears the entire territory like a bulldozer, at the speed of a racing car.
I didn't play the campaign, only this map separately. For strategic gameplay, this map is one of the best. It requires knowledge of the economies of the castles, plans for their development. Good interaction of heroes, clearly defined tasks for each. And, despite the fact that the player has a large group, there are no useless characters there. The map is fast-paced; it is not suitable for those who like to pick their nose for a long time before doing anything.