The best spot to knock out these kills is during the Dark Star mission. However, you need to make sure you grab this weapon for free during the previous mission, the Church. Make sure to exchange one of your weapons on the left or right (not the middle one, because it is swapped out for the flame thrower during this checkpoint, although you can get it back afterward and use a later checkpoint to farm your kills if you accidentally forget and put the Hellbreath in the middle weapon slot).
I've created a full Enlightened playthrough video showing almost every Moral Point in the game as well as pointing out items of interest, such as this achievement.
You can see the checkpoint I'm referring to in the Dark Star mission at the 1:48:04
mark in the video.