By the time you meet the cabal you should have 3 of these already.
The same trick applies here. Shoot the enemies in the head, (a little harder with the enemies with the shield). While you have to be in a mission (and not patrolling) you do not have to be in the mission area, if you're having problems doing it during a mission, just find some cabal, head shot those you can, drive away for a bit, come back and they should respawn, and you can kill them again. You do not need to be in a mission only area, just on the way to the mission.