Kill 2 enemies with one Railgun Slug shot through a wall
Introduced into this DLC is a new weapon called the Railgun. You come across the railgun after receiving it from Sarah after completing the Hagar Caves quest. The Railgun has three different ammo types;
You can select which type you want to use by selecting and using the to navigate. With the Slug rounds, the Railgun offers an x-ray scope to view enemies through walls. It's going to be tough to line up two enemies at once (let alone through a wall), a good spot it right in the beginning of the Refinery level. After progressing a short way through, on the right hand side of the large corridor you're in, head down that way and there will be three enemies all talking with each other. You can take the shot through the wall next to the door.
Easy. First group that you encounter in the refinery Move towrds them and to the right is a corridor where you can hide behind a wall. Line up the shot when the moving guard stops to talk to the other two.