Go to quick race, set the AI to Exhibition, race type to Magnet Mayhem and choose a nice long track. Start it up and what you have to do is stay with one person at a time and try to take them out as fast as possible either shoot in front of you or if you are in front of them shoot backwards but don't speed of when you are, maintain your speed, then rinse and repeat 2 more times and then the achievement should pop up.