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

In my playthrough, Kolya simply fled the battlefield, just like the other players, and couldn't even gain experience. The resurrection spell appeared in the city of Narghorad, and I only used it a couple of times to resurrect phoenixes at the castle.

You don't really need to take phoenixes; if you destroy all the spectral dragon units, they are located in the same mage tower. It's much easier to fight Kolyan with the dwarven artifacts. And to prevent him from fleeing, you need to obtain the "Chains of Inevitability" artifact.

Added 4 minutes later

I have a question. I've read many times that you need to constantly resurrect your army in battle, otherwise you won't win. But I don't have any resurrection ability. And can elves have it, and how do I get it? I don't remember being offered any resurrection when my hero leveled up...

Resurrection is a light magic spell of level 5. To obtain it, you need the "Artful Light Magic" skill and a magic guild of level 5. The highest chance of getting it is in the "Defense" mission; you can build a level 5 guild in both cities, and you are guaranteed to get the spell in one of them.