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

I captured a Castle through a portal, then moved to the Dungeon in the center. After that, I abandoned that plan, quickly caught Nicolas (there were Phoenixes), and killed him.

How can I quickly obtain Phoenixes? The whole problem seems to be with them. I understand that once you get them, Nicolas and his followers stop roaming the map and gathering an army; instead, they sit quietly and wait for me.