Ahhh great idea:D, you know anyone who hosts?

Well you used to be able to do the elevator in Karachi and then go under the map and continue to drop, by holding down on the analogue stick you could complete Marathon and Lightweight within a minute. Sadly the elevators were patched.

Another thing that you used to be able to do was go in into a speed lobby and then just run around constantly. I am not sure as to whether or not there are still speed lobbies around now but by doing it by this method, it took only a few games to complete Marathon and Lightweight.

Your best bet is to play a long lasting game mode such as Domination or Headquaters and just go to a far off section of the map you are on and run around a small area continually.