Turret Expert: Kill 250 enemies using turrets.
How to Get the Turret Expert Achievement
Start the Bunker Busters Raid on any difficulty with or without bots, then continue the raid until you enter the bunker.
Once inside the bunker, instead of going downstairs take a right to a room with a turret overlooking the entrance.
Enemies will try to enter the entrance, but you just have to shoot them before they get in.
The achievement will unlock when you finish the Raid, not when you get 250 turret kills.