Archon of WarG 15

You will need to go use a skill point to unlock the "Takedowns" skill in your skill tree, which will allow you to press right analogue stick on stunned or downed enemies to perform a takedown. You can track your progress on this achievement under the "Merits" tab in the menu under the "Brute" merit in the "Combat" area. Perform a total of 100 takedowns, and the achievement will be yours.