Easy "Coup De Grace" achievement
During the Slone Boss fight in Mission 9: The Fold Weapon, damage Slone until she has almost no health remaining. Then, run up to her and perform a melee attack (when the yellow health bar starts to show) to perform a special takedown animation and get the "Coup De Grace" achievement. Note: This can only be done against certain bosses since some bosses explode instantly and you cannot melee them.