Easiest way to get this I feel is to start up any capture the flag match and grab a motorcycle from around your flag then drive it into the enemy flag area, hit X to switch to the back seat and try to mow an enemy down with your main weapon before they kill you; should be at least one person camping near the flag at any given point.
...or you can actually try to get it as they intended and have someone drive you around...but that takes forever most times.