I just finished playing the map as a "hero." My demon was barely moving, dragging itself along as much as possible. It barely made it to the final castle on day 79. It was so difficult, whew. In the Necropolis castle, I had been gathering forces with increased recruitment for several weeks. For the final battle, I used my usual troops. I won without any major problems. There would have been even fewer problems if the map had common "light" spells. This is the second time I've played, and the second time there are no light spells in any of the light castles. It's some kind of conspiracy.
So, it's definitely not worth changing troops.