Here are general tips for playing the Chaos Champaign in Warhammer: Mark of Chaos, on HARD Difficulty. At the end of Chapter 1, Mission 6, we chose Nurgle instead of Khorne. A list of all our Warhammer: Mark of Chaos articles can be found here.
We won't be hand-holding you through absolutely everything, but instead just give tips on situations that we found tricky or that you might not have considered.
Chapter 4: The Rise of Chaos
Mission 4: A Brave Stand
For a late-game duel, this is remarkably easy because again, the enemy only has basic armor and magic resistance, while your hero can have as much as 10 Armor. Thorgar should be able to win this without any skills. Go have a coffee or something and come back later.
Before entering the next mission, rearrange your gear amongst your heroes. You must have Thorgar and Kasqit for the final mission, and you can choose two others (but really, there are only two choices -- the Chaos Undivided Sorcerer and the Grey Seer, as they are the only two that can resurrect lost models).
Some gear are specific to Chaos or Skaven and cannot be traded to the other faction. Heroes will be able to attach to the regiments of any faction as long as attaching is allowed.