Make it past the conveyor belt without taking any damage from the turret
The conveyer belt level is on Chapter 15 of the single player campaign. As soon as the level starts and you exit the vent, do not move. To your right is the turret that you have to avoid. Kill the enemy directly in front of you and peek out just enough to see the conveyer belt controls that you have to override. Overriding the conveyer belt moves along a box that you must keep between yourself and the turret as cover.
You have to go up the room once, then back down (killing the enemies along the way) with the box as cover, utilizing the conveyer belt controls to keep it moving along the way. When you reach the second room, you can shut off the turrent, and when you finally exit that area, the achievement should pop.