The objective of this achievement is to destroy the floor that an enemy is currently standing on. This works for both mobile enemies and turrets.
When you see an enemy standing on a roof use an explosive to destroy the floor and make the enemy fall. The enemy must be killed by fall damage and not your actual attack.
I found the easiest place to accomplish this is on the first ADVENT Blacksite. Stay concealed and move towards one of the many turrets on the map.
Position your team so that you can perform multiple attacks on your turn. Once you're ready damage the turret with guns until it has 3-4 HP remaining
(Weapon damage - turret armor = actual damage).
Finally use a grenade on the turret to destroy the ground from beneath it and watch it fall.