So it's quiet at work so I've done some research. It seems the "Rat Run" consists of the following: 2x Rathma Necromancers with Singularity/Skeletal Mages (build). They use Reaper's Wraps as their primary method of getting essence (health globe restores 25-30% of resource). 1x ZeroDPS Necromancer (build). High CDR build, with two key roles. Keep Land of the Dead (Frozen Wastes) up ~100% of the time to freeze mobs, spawn lots of health globes from corpses. 1x ZeroDPS Barbarian (build). HIgh pickup radius, main roles are to keep Ignore Pain up on the DPS Necros, and to pick up all the globes (with that increased pickup radius gearing) spawned by the znec, to power the DPS Necros. Both ZDPS builds have a combination of gems (iceblink, efficacious toxin, etc) and abilities to debuff mobs to decrease damage dealt and increase damage taken, but this is normal in most standard zdps builds. It seems there's a little variety on exactly the build that each uses: the necro builds I grabbed from reddit/r/diablo3necromancers and the zbarb build was just google, but basically the idea is the znec scouts around and spawns globes, the zbarb stays reasonably close to the DPS Necros to keep Ignore pain on them for Damage Reduction as well as picking up all the globes. I've also seen references to pull barbs who actively scout around and pull mobs back into a group, but it seems this is for much higher paragon (1500+) players.