This achievement is obtained in the "Clash In The Clouds" DLC. The video is from the first level called "The Ops Zeal".
I upgraded the Bucking Bronco vigor to be able to lift 2 enemies just to make it a little bit easier. Wave 6 is where you will have an enemy with a rocket launcher in the middle of the map on a raised platform firing down. Go to the far right from where you spawn into a room. Kill the enemy in there and wait for a few guys to rush you. You may want to weaken them a bit before you get started but it's not really necessary. Simply using Bucking Bronco to lift the enemies and stand where you have direct line of sight from the rocket enemy. Fire when the rocket is in between the floating enemies. If the rocket hits and kills them directly it doesn't count for the achievement.