Load Chapter 11 U-Boat and you will already have the shrapnel mode active so don't change your gun or anything like that. Walk forward a little bit and there is a guard in front of you.
Shoot the ground a little bit below his feet. Then kill everyone who comes into the room after the alarm sounds the same way, reload checkpoint if needed. I got 6 kills the first try and the last 4 after reloading the checkpoint.