My Night Job

You increase your multiplier as you kill enemies. You must kill at least one enemy within about 5-6 seconds after your last one or else your multiplier will reset back to zero, so you will want to keep killing at a fairly consistent rate. Once you get it up to 99.9%, then just make sure you keep killing to keep it up there for two minutes.

The easiest way is to find adjoining and close rooms that are being stomped or filled with enemies and try to kill an enemy, wait 2-3 seconds and then kill another one, then repeat so you don't run out of enemies too early. Once you make it two minutes with a 99.9% multiplier, the trophy will pop.