A very easy strategy for this is to play until (or reload) the chapter "Good Times Club".
Upon entering, go upstairs and you should enter a large arena which looks like a theater with a huge stage up front. You'll have a speech from Fink. During this time, lay down some Devil's kiss traps (helps if is upgraded) along the stage. There are salts around via tears, if needs be. As long as the long talking sequence is over, 5-6 enemies should come through and kill themselves immediately with the traps you've laid. This will kill at least 3 people, and it might even kill 6 and count as 2 of the 5 if done right.
Either way, reload the checkpoint and you should see your traps already assembled (unless you laid them RIGHT before they entered). Allow them to die, and rinse and repeat 2-4 times until you get the achievement.