This is story related. In Chapter 7 you will reach a point where you see Leslie in a cage. You will free him and open a door for him. He will freak and lock you out. Here is whare the Keeper will spawn. You will need to kill this keeper and tun the 3 valves off and exit the area to achieve this.
There is a 2nd achievement that can be earned here. It is for completing the section here killing only 2 keepers. This would be the 1st Keeper and the 3rd one. When the 2nd spawns you will want to avoid him. You can use shock or Ice bolts if needed on the 2nd and 4th instances of the keeper.