Ambush is a skill in the Stealth tree, to even buy this you need to have bought
Low Profile 1 for 1500 Green Gel so you can then buy
Ambush for 3000 Green Gel.
For this skill to take effect you need to be in cover and wait for an enemy to come towards you. When the enemy is close enough you'll see the Stealth Assassination symbol appear above their head. Press
to Stealth Kill around the piece of cover and the achievement will unlock.