This achievement requires the Insurgency DLC. Difficulty does not matter, so might as well make it easy and do it on rookie.
This is done in the 2nd area of the Portland ME map, Hunter game mode. Follow the video guide by kardsharpGaming below, and it will show you a good path to take to get to the washroom. Then it's just a matter of making sure when you kill guys that they're actually in the washroom area, and not the hallway.