Connect a guest controller, and start a private Casual Execution match (on Boneyard, obviously) with all weapon spawns set to Frags and Friendly Fire "On". Once the game begins, grab some grenades, kill your teammates and then kill the four bot opponents.
After that, a one-minute timer will start. Find your Guest (he shouldn't be far away), and aim a grenade on him from a reasonable distance (50 feet is not particularly far). Once you're lined up, cancel the throw and then "blind fire" the grenade. It should go exactly where you aimed it. Once the Guest is killed, the game will end, the ribbon will display, and then the cheev will pop.
This worked for me on the first try.