I use the Auto-Rifleman Class and select the MK48 LMG. Then all you do is play Co-Op Territory on Oldtown 1 base with Infinite Enemy Density.
Lay down and Camp on the Left staircase. Put the Drone right above the bottom of the stairs. Point your Reticle at right where there heads will be . Pull the trigger like a single shot rifle. You get 100+ kills every 5 minutes. and after about 1500 kills you will get the Sniper achievement.
So it only takes about 1 hour 15 minutes and you are also getting the most kills the fastest towards your Heavyweight achievement.
The trick is to only kill 100 people then let them kill you and take the base. For some reason the first 100 kills take only 5 minutes. After that you will not have the same kill/minute ratio.