Quartermaster Diamond is a veritable nightmare, even if the difficulty isn't set to N!. I'm gonna bet it's not nearly as bad in .992 (and probably moreso in the beta) but trying to kill everything in 5 minutes on UV is H-A-R-D. You'd think it would get easier over time. And it CAN, but the exponential build you'd consider using (Int2/MCe) is particularly hard to pull off, not to mention gives you no Fin/HR/Rel benefits of speed. On the other hand, Fireangel may keep you alive, but it'll just get more and more difficult to kill everything in those five minutes. My opinion is HR -> Iro -> HR -> Iro -> Iro, THEN go for MFa, so you can abuse your health and take as much damage as possible (note that Iro is taken in anticipating late-game as opposed to TaN for an easier late-game). If you want to play an Int build, it is theoretically easier but practically a lot more grinding on the early levels.
Fortunately in .992 the maze levels are opened up, so the only real issue is the Maze of Rooms. Int build will tell you the shortest path to reach those crappy enemies hiding in the corners, while HR will let you view them sooner. Do at least partial HA for the experience (clearing some waves and refusing the others won't hurt your kill rate), but if you can complete it, the RL will come in handy for blowing open walls. The rest is mostly how well you can play and guess where the enemies are hiding (or, if playing an Int build, how to minimize time spent walking). It's a tough one. I managed to hit City of Skulls way back in .991 (unfortunately I was forced into it due to time constraints) and barely didn't make it. That was an MFa build, and it's probably the one I'll be using when I aim for it again.