I think this was one of the easiest adventures we've ever done. The Wife's a fan of using all five heroes whenever available, so taking out twelve monsters was a snap. By focussing on killing monsters, we got plenty of XP to cancel the occasionally problematic encounter cards. The special abilities of certain monsters that are usually quite dangerous, like the keyword Sentry monsters and the Legion Devils, actually proved really helpful in speeding gameplay along. We took out twelve monsters, but I think we could've handled 24!
Maybe this is one of those adventures that doesn't scale well, and would be much harder with fewer heroes . . .