Balaho's Most Wanted
This is a grunt they are vicious creatures, may be small and run from you a lot but they are still simple to kill. You shoot 100 of them and you will obtain this achievement.
Killing any grunt will go towards this achievement, any shape or form.
Note: If you kill the elite usually with them then they will freak out and start running away from you or around in circles. Can make them easy targets or annoying when trying to head shot them.
Below is a (somewhat) educational and halarius video of the grunts done by Machinima.