map: manor
gamemode: deathmatch
Gamemode Settings:
Score to win: 5 (to ensure you get it-might take a few tries).
weapon of choice: i used the blunderbuss but i'm sure any weapon will do.
This method is with a second remote. Find any accessible vent (grey openings with white lines indicating a sucking motion ;) ). I used the one in the green zone, at the bottom of the stairs on the bottom level on the right. Put the second remote at the very edge of the vent right before being sucked away to the end of it. Now, with the main account going for the achievement, walk into the second remote and keep going forward into the vent. If you hadn't figured it out by now, walking into someone makes them move forward and makes your remote shake. As soon as you see both screens begin to fly through the vent, shoot the second account in the back for the achievement.
The reason why i recommend the blunderbuss is simply due to its wide shooting range, without blowing up in your face.