I have seen this mission from both sides now.
Completely breezed through this on my 17 P/W but today decided to bring my N1NJAAA from Cantha over just for something to do. Fair enough, I didn't level my hero's beyond 15 before my first attempt but after failing and failing and failing they were around 18 by the time I finally got through it.
After suffering through that god knows how many times (A/Mo but the Mo is just for show) I decided to bring some regen skills to help keep me up (despite running both monk heroes).
I found going for the Spotters first made things easier for me, maybe I'm just picky but I always leave the bosses until last, most hero's and hench can stand up to getting pounded by them. When going for the bonus there are a couple of spots where the Bowman can hinder your progress to the Spotters, in which case : Command your group to the Bowman front line and make a hole through.
Seriously, 5 or 6 attempts at that on the 20 Assassin. Maybe it was the low level heroes or my oustanding ability at suck, who knows