The best way to do this, IF you have 3 friends + yourself, is set up a:
The person going for the achievement is on a team by himself and all the other people are on the other team.
The person going for the kills should capture 1 flag, get the red Doubler, then go back to the other teams flag and have people run to you and just shoot away for the next 28 or so minutes. You will get approx 500 kills per match depending on how fast your friends can get to you.
The reason you capture 1 flag is so you don't go into overtime. (I noticed on a couple matches that we went into overtime and I only got a few kills instead of the 300+ I had done.) Big waste of time....
Another tip: You use the Doubler to get "one shot kills." If you are running low on ammo tell the people to pick up guns and equip them for you before you kill them. That way you can pick them up when they die. I preferred the Rail Gun and Rocket Launcher myself.
If you find this solution helpful please don't forget to vote Positive. :)
Thank you and Good Luck!!