Requirements: Time Bend 2
Recommended: Upgrade you Crossbow so it can reload faster
Method:
At any level as soon you can get Time Bend 2 and Upgrade you Crossbow so it can reload faster, you can just use sleeping darts on 4 guys, line them up in the same place, then bend time, and head shot them all as fast as you can.
P.S.
Try to get guys with no helmet, you may hit the helmet and have to shoot again.