Walkthrough tips, Campaign discussion [Heroes 5: Hammers of Fate].

I apologize for bothering you, but I have a problem with campaign 1, mission 5. I managed to get through the "Garrison of the Gates" (where Duncan is) with what I had. However, I can't reach "Gustav's Bend." After about two days, a red unit appears and chases me, even if I run away. It's as if the entire map is revealed to it. I've tried training units and changing the difficulty level, even to recruit!
PLEASE HELP!