I Found an easy place to achieve this, It's during act 7 chapter 2. When you reach the point where you can eject a second group of enemies, immediately after there will be a hall way with a gun turret, use your shotgun to send the enemies flying down the hall (don't kill them), now run to end of this Hall and I door will open and more will come though. Now, since I was in the middle of them I aimed the leash at my feet and activated the thumper and smashed them all into the ceiling.
BELOW is the video from xbox DVR showing this, And I hope it helps.
Score in video was 8x400=3200, best from multiple tries 9x450=4050. Min for passing grade 7x350=2450 based on Flyswatter skillshot score.