BioShock Infinite

Here is a quick easy way to get this done and they can actually land on a platform below.

This achievement can be obtained in any level of the "Clash In The Clouds" DLC. The video is from the first level called "The Ops Zeal".

I upgraded the Undertow vigor to have a longer range of use.The very fist wave is the best time to do this since the enemies are the weakest. make sure to clear the rest of the enemies and leave one so you can get set up in peace. I simply shot an enemy off a high platform onto a lower one but not low enough to kill. I then Held the vigor button to use undertow to pull the enemy back to me. Finally I shot the poor enemy one last time for the kill. You don't actually have to shoot the enemy off the edge of the map where they would fall to there doom if you didn't pull them back.

I found the easiest way to get this was in the second arena, "Duke and Dimwit"
There are less heavy hitter enemies throughout the waves and there are more of those air carriages so you do not have the railing to deal with. Just make sure you have enough Salts to us undertow three times. Knock someone off the airship, grab them before they fall too far, then hit them again.