You should be able to get these easily, the trick is to find a mission that works, or a location. The moon has the best population of Hives roaming, so choose a mission on the moon, and then find a group of the hives on the way to the mission (you do not need to be in the mission only area to do this)
When you find a good group, head shot all of the enemies you can, then drive away from the group (You want to go a decent distance, but probably shorter than expected) and then return to that location. You can head shot the enemies again and keep returning. In the mission only areas, the enemies do not respawn, on the way to the missions, the enemies respawn ad nauseum.
Continue to do this until you got your trophy.
NOTE: you MUST be in a mission, patrols will not unlock this trophy.