[Shoddy]

Slave Pens and Mechanar are generally regarded as good first choices. A 3-badge Mech run (first boss and two mini-bosses to open the elevator) can be pretty quick once you get the hang of it. In Slave Pens, you can skip a lot of groups so it can also be fairly quick, but the dual Defender groups and the large groups later on can be tough. Of the two I'd probably recommend Mech, not least because the corpse walk is much shorter.