First boss in DTK is cake for casters. Prince sucks in UK, however in UP the first boss is super easy. I just burn her down the whole time I dont even attempt to save any of my guys. I didnt even know that the second boss in UP existed (thought Skadi was the 2nd) so Im not sure how easy he is for MBing. Gundrak, Moorabi is easy.. I dont even interrupt him I just go to town and make sure to heal my tank more when he transforms. Also Eck is super easy once you understand the fight. I really like Gundrak because there isnt a whole lot of trash. I usually clear it in this order: Moorabi, Eck... then I exit and fly over to other entrance and kill Sladran and then go to 4th and then the final boss. It might be easier to clear the trolls than the snakes, but I havent tried.

edit: Oh yeah, CoS is really really easy as long as you take your time and dont freak out that you are not going to make the Infinite Corrupter. That will take some good gear and practice to make on time.