You can only get this achievement as Taggert. While on the level "Breakout" at the beginning of the level, crouch and go into stealth mode. Walk-crouch to the enemy in front of you and when it shows "stealth kill" click left on the mouse (or
if you have that PC controller adapter) if you stand and stab him, then it counts as one stealth kill. If you slash at the enemy, then it does not count as a stealth kill, you have to stab them.
Go into the pipe next to where you killed that guy and try and kill 1-2 more enemies. Once done with that area, let the rest of the guys in that area, or the next area kill you. When you repawn, do this over again until you get the achievement.