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Sorry, guys, I just panicked a bit. I completed the mission! Steel-clad cavalry, Knights in shining armor, Angels, and cold steel crushing everything in its path! In the end, it was just an avalanche. The Demons were begging to go back to Sheol! Great feeling after a successful completion, especially after struggling with it. But there are still other battles ahead.
As for the tactics, I focused on magic defense as much as possible at the beginning, and I played without the Phoenix Amulet (at first, then some enemy hero brought it with him). I guess I was lucky at the beginning; the first two random heroes from the portal (red) were warriors, and I just crushed them (because mages in the early stages were a pain). So, after quickly capturing the first town and two forts (carefully collecting all the nearby resources), I immediately rushed downwards (underground) towards the Orange player. (Yes, maybe there was some trick to it; I didn't capture the second town nearby, nor did I go to the Deer player). I dealt with the Orange player easily, occasionally getting distracted by the aforementioned random red players from the portal. And then, basically, that was it. I came out of the dungeon, finally captured the second town nearby, repelled the attacks of the red players, and started BUILDING. After a week or two, I broke through to the Deer player. By that time, the infrastructure in the castles was already providing a good weekly replenishment of the army, and the portals allowed me to repel attacks relatively easily (I bought a second hero for convenience; he had 1/3 of the army, and I only merged them when strong enemies appeared). I had to buy a lot of resources, and I tried to build Angels as quickly as possible, and I never regretted it.
I apologize for the rambling, but honestly, this mission was really difficult at first, and now I'm just feeling a bit euphoric