Wizard – “Haven” [Campaign 6, Mission 4]

I can't seem to beat Isabelle. I've been fighting her for 4 months after capturing and rebuilding all the Necropolis and Haven castles and buying troops almost everywhere. By that time, she has 1600 monks, and they destroy all my troops. I have more troops of other types, but it seems that even though Isabelle has only one castle against my eight, the growth rate in her castle is not the same as in just one.
And where is the second Sylvan tree? I found one behind the "Northern Cross," but I can't find the second one. I've explored the entire map, but I don't know where it is.