While playing through the stage "New Blood" you'll wander through a sewer/maintenance tunnel. You'll have to hold X to turn a wheel on the wall which in turn raises a barrier. You have to then activate your time powers to get under the barrier before it lowers again.
Immediately after this you'll be climbing out of the tunnel into what looks like a park. This park has a number of enemies in it.
All I did was climb out of the tunnel, kill the first enemy I saw to get their attention and then drop back into the tunnel. Use your radar and wait until you see at least 3 of them clustered nicely at the top of the ladder. Climb up, Stop Time and shotgun 3 or more of them. I only had to restart once while unlocking this achievement.