This achievment can be unlocked in the first level "
Osaka Castle".
About half way through the level just before you are introduced to the shadow mode concept you will have a long hallway with 3 gaurds within it. Kill the farthest guards from the door and using "
Mugen" fling one of the bodies at the remaing guard by the door and the achievement will pop. (
Video guide below, start at 37 seconds in)