BioShock Infinite: The Complete Edition

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.

The guides below detail using Devil's Kiss at the Good Time Club chapter. If like me, this was not counting for you, use Bucking Bronco. The trap does not need to kill them, only affect them. Using this vigor also allows you to hit the same group of people multiple times without having to reload the chapter.

So the way I got it easily is when you fight Lady Comstock she will resurrect the dead usually 4-6 enemies will spawn. They float in the air for a few seconds before landing which sets up plenty of time for you the throw a trap under her. I used Devils Kiss traps which eliminated them all and made her summon more of them frequently. Hope this helps if anyone is having trouble with other methods!