In Mission 5: Chapter 2 - Effect And Cause, there is a Powler in the first room after learning how to time travel (main objective: "Take Anderson's wrist-mounted device"). Enter the cage in present time, then travel to the past to see the Powler. Kill it to get the "It Was Coming Right For Us" achievement. If you have already completed this mission, select Mission 5: Chapter 2 at the mission selection screen to start at the point when you get the time travel device so you do not have to replay the entire mission.