Ranger - "Higher Vampire" [5th Campaign, 5th Mission]

I don't understand why people are having so many problems with this mission. When I played through it, I didn't bring any phoenixes or undead troops. I defeated everyone with troops from the first two castles. True, during the exploration of the map, I eliminated all the neutral ghost dragons, so Kolya died easily in one hit. However, there were no armies with 10,000 skeletons; the maximum, as far as I remember, was around 6,000, and that was only with one hero.
PS. With this setup, I think you don't even need to complete the quest for the armor to save the artifacts. But I still completed it because I already had enough worthy artifacts.