After you've heard about Swifty (when you took the elevator with Vinnie, in your house, after the very first mission) you'll be in an alley and you'll open a door with your demon arm. As soon as the door opens, grab the javelin (pool stick()) on the container to your right and throw it () at the enemies as soon as they're aligned.
If you use the Swarm ability, it becomes a lot easier as the enemies are stationary. I did this in the final chapter where there are masses of enemies coming at you, got it really easily without even trying.
Even easier than that, on the level "Homecoming" after defeating a large amount of guys (after seeing a picture in the lobby....you'll know when you get here) you hit a checkpoint by a upgrade rift-thing. Backtrack to the room you were just in and grab a pole by the stairs. take it with you back into the room with the rift. Go up the stairs and look down in the hole. There are 3 bad guys hanging out with low health. Line up your shot and let it rip. Bam! Done and done.
There is also a relic right next to where you stand (2 of 2) for more clarification of the spot)