This is a toughie. The easiest way to do it is to have three friends make a party of Arena CTF Endurance and search, while you search being alone. Take the doubler, go to their base, and have your friends keep lining up by the flag so you can kill three with one shot, you will receive a rampage when you kill all three in one shot as long as you are accurate, and will receive 2 excellents everytime you do so. It's possible to get this achievement in two games if you have good quick friends! If not, just keep doing it until you can get 500. If you don't have any friends online, play deathmatch average or endurance and just try to use rockets or grenades for double kills.