Very self explanatory,
Get 10 kills in a game with the Gromato Launcher
Make an online game (with bots) and uncheck every venue except for Darkside. Change the game to 10 minutes in length so you have plenty of time to do it.
There are plenty of gromato Launchers available, so all you have to do is shoot it directly at , or bounce it at , a farmer, and they should be killed in one hit.