New era.

What difficulty level did you play on? I'm mildly confused by your story about clearing the map with Zehir. On Hero difficulty, all the starting areas are impossible to clear with Zehir, period. You can only take one stack at a time, like the 100 Gremlins guarding the Gremlin building or the 50 Dancing Blades guarding the gold. All other stacks are much stronger than these, and they're also upgraded. It's simply impossible to clear with Zehir. The same goes for Gothai, as his army won't be enough to clear even the basic weak neutrals in the center of the map before the inner garrisons. And the guards at the Orcish buildings are even stronger than those in the center. In my game, they were guarded by 30-40 Gnomish Priests, 100-120 Ghosts, 5-6 Cyclops, and others. So, I cleared the entire center before the inner garrisons, but I didn't have time to reach the mines. I also cleared all the enemies around Zehir's territory, except for those near the Orcish buildings and inside the territory. I cleared half of the forest to the right of the center and half of the area to the right above the forest, up to the unit upgrade building. This took me a month and a week, and by that time, the Demon hero reached the central castle and easily killed me. None of the reinforcements arrived; I didn't even understand what I needed to do for them to arrive. The game says that you need to capture the outer garrisons, but how do you capture them in the first month? It's impossible. As a result, they died without even appearing on the map.
Overall, I've come up with a few ideas on how to improve the starting strategy, but even if I gather all the troops of one hero and clear Zehir's territory (I'm not even considering the Orcs, as you need 6-7 units in the army to clear them), I still don't see a clear advantage over the first Demon. I'll probably try on Veteran difficulty, with the Anlak hero. I suspect that the campaign might be conflicting with the EVE mod, which enhances the AI and units on the map. I don't remember all the buffs of this mod, as I downloaded it a long time ago. I remember looking for ways to make the AI move faster and the neutrals stronger. Maybe that's why the neutrals are so tough in this campaign. Although, I downloaded other campaigns recently and successfully completed them, and I didn't notice anything like this; I would even say that they were very easy.
And yes, by the way, Zehir's ability isn't working for me. Elementals don't appear after a unit dies. So, something is wrong.
I've completed all previous maps and campaigns, including Master difficulty, with this mod, so I still think the problem isn't with it. There are no other campaigns in the folders, only 3 multiplayer maps, two of which are from Master difficulty and one from Ganrytor, where there are no mods or special scripts.