Questions about the gameplay of Heroes 6.

I have a question about the walkthrough. I encountered a problem while playing the Necropolis campaign in Heroes of Might and Magic VI. The Faceless offers Anastasia to visit his nightmares, and only then will she be able to find answers to all her questions. The problem is that I was given a small army, and at the end of the dreams, I fail the mission. Can you give me some advice on how to complete it? Thank you in advance.