Mission 5 - Chapter 2: Effect and Cause
Recommended Difficulty: Easy
As soon as Chapter 2 begins, you will earn the ability to "time travel" between past and present. Directly after attaining it, use past time to gain access across the hall to the research lab with caged prowlers, noticing the prowler behind glass in the back. Go to present, crouch through the glass, and switch back to past, immediately killing the prowler to gain the achievement.