*Ambush*- "Down an enemy without taking return damage"
This achievement is self explanatory down an enemy with out taking damages
this means when you down someone you need to have 100hp right when you down them.
the easiest way to get this achievement is with Saint or Ronin in the game mode Strikeout as they have automatic detonator when an enemy gets to close that will down them instantly if they don't have the perk Tenacity equip, just place them around the objective and hide until someone hits it.
If you like this game you will get it just passively playing