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

I was playing on Recruit difficulty, and while I kept Phoenixes in one of my castles, I couldn't defeat Nicholas. He kept running away, and on the next turn, he would appear in another location with a sizable army. However, he didn't venture into any dungeons; he just destroyed my "mercenaries" as he pleased. Only at the end, when I finally figured out how to move the Phoenixes out of the castle, did he enter a dungeon himself, capture a city at the bottom left, attack Fayedain, and unfortunately, was "defeated."