You need to get through the labs without setting off an alarm in mission 7 (One Bullet). The very first scientist will spot you and set off an alarm. This is inevitable, but won't influence the achievement. Speed (and shotgun for close quarters) is your best friend for this. Each time you open a door, you must quickly kill everyone in the room before they hit an alarm (located on a wall). If they spot you everything is fine, they are just not allowed to use the alarm switch. Rush in with a shotgun on Recruit difficulty and kill them quickly. If they reach an alarm, it will begin a countdown. If you hit the switch () to cancel the countdown, the achievement will still unlock. If you're sure you've set off an alarm, pause and select "restart checkpoint" to try again. The achievement pops when you've completed the mission.