This achievement is rewarded for killing no more than 2 Keepers during chapter 7.
The section will start after freeing Leslie from a cage. He will panic and run locking you in a room. A Keeper will then spawn. This is the first one you can kill. The easiest way to down a keeper is 2 Explosive Bolts. I do not recommend the magnum as you are better off saving the ammo for bosses.
Once you kill that keeper. You can make your way to the first valve. If you are quick enough with your key mashing, you can close this valve before the Keeper reaches you. If you can not, then you can use a Shock Bolt or Ice Bolt to paralyze the Keeper. After that Valve is shut, you run straight into the room with the Circular metal door. You can then close the 2nd Valve which will spawn a Keeper within the room you are in. It will only spawn once the valve is closed. This is the 2nd and last Keeper that you can kill. Once he is dead you can leave the room and head to your left. You will see a Keeper destroy a valve.
Head back to the pathway towards the first room and you will see at the end of the path a Valve on a counter. It will be highlighted in blue and easy to see. Grab that and wait for the Keeper to catch up to you. Once he is near you can freeze him and head towards the first room. If you turn left upon entering room 1, there is a path straight towards the room with the 3rd valve. Simply attach it and close it. Then all you need to do is run to the exit.