Most people. At least, most who have tried which is… well, probably still most people. I got it the first day of the content being out, on my second trip through the missions.
Just come up with an imaginary grid and run STRAIGHT LINES from East to West, one side to the other. Move just a little bit South when you hit the wall and keep going until you’re certain you’ve cleared everything. Then just hang around near the respawning hyenas and clear them out until you have the achievement. It’s… really simple.