Hey Father, Watch This achievement in Far Cry New Dawn

To unlock the Hey Father, Watch This achievement in Far Cry New Dawn, you will first need to progress through the story to a mission in Act 2 so that you are able to purchase the Leap of Faith gold perk for 7 perk points. Once acquired, this will allow you to press A whilst in the air for additional jump height -- basically a double jump.

The easiest places to find a stationary enemy is at one of the game's 10 Outposts. Simply sneak into the Outpost, get above an enemy, double jump and press the right stick to perform a takedown.

EDIT: Thanks to PatchyAmac who mentioned in the comments that you can do this by sprinting at an enemy, press A twice to perform a double jump, and then click the right stick to perform the takedown.