On the 2nd level in the game Back Again (The first real level where you have access to your time powers) you will see an explosion and walk through a cement pipe with some friendly soldiers on the other side fighting enemies behind a fence. It makes an auto save in the tunnel. One of the friendly soldiers will die and drop 2 Clutch Grenades and once one of the enemies either jumps down from the wooden planks on the far wall or is standing up top shooting walk out of the pipe stop time and stick him with a grenade. If you turn to your right there should be another enemy soldier already on the ground that jumped down and you can stick him as well. After they both die you can reload the checkpoint and keep getting those 2 kills over and over until the achievement unlocks.