You will probably get this trying to get the Supernova achievement. On the echo "Crash Site" you need to kill a burnout using the chopper that's falling out of the sky. I'm a fan of the drill gun as it's easy to rack up multi kills with it as well so it's pretty easy here too. I shot the first one and it grabbed few on it's way to the blades. If that doesn't work you can also try thumping them first.