This achievement is for getting two kills with one shot from the Balistazooka. The easiest/quickest place I found to do this is near Fallgrim Tower. Exit the tower via the doorway by the Sester. Take a right and you’ll find two enemies sitting in the path (one playing the lute.) Move close enough that they both get up and begin walking toward you. Once they are lined up take the shot. Should you miss there is a group of enemies by some campfires if you take the path on either side from this point.