The first step to unlocking this is to unlock The Reaper
in the garage. To do this, liberate the Kellett Cattle Co.
Outpost in Holland Valley, and you should now be able to spawn it from any garage on the map.
From here, it's as simple as spawning the vehicle and finding 5 enemies to run over. You should be able to do this easily on the road or by finding a new outpost.
You will still be able to find enemies after 100% all of the story missions and side quests, although their spawns will be much more rare.