Another solution that worked for me is in Shantytown when you're trying to rescue Grim.
It's the part when he tells you it's a trap. They'll be 4 enemies who zip line across 2 cables one after another.
Use your crossbow and try to get head shots. Once you've taken your shots at the 4 enemies, regardless of whether you hit or missed them, just jump down the platform to your death. Don't kill anyone else in that scene. As soon as you die, you'll be able to play the scene all over again.
These kills are cumulative even though you're replaying the same scene over and over again.
So you can keep playing this same scene until you get the achievement.