I found this easiest to grind in the My Gory Valentine movie, Scene 12: Slickenside Security (2nd to last one) as you can pick up 11 kills each time (UPDATE: only 4 kills counts towards the achievement, thank you jojoborne for pointing that out). The solution is:
Right, Left, Up, Right, Down, Down, Up, Left, Down, Down, Right, Right, Up, Up, Right, Up, Left, Down, Left, Down, Right, Right
When you complete and move on to the next scene, pause and press up to jump to the bottom of the menu. Click Back to Video Store and repeat the process. Even though the solution has a lot of steps, it is easy to memorize it once you do it a couple of times.
Kills from Faces of Killed 3 does not count towards this achievement. While the achievement itself does not track, you can go to the Official Club Page and go to Stats to see your kills (wish I knew this sooner). I checked my method this way and my kills only went up by 4.