The best place I have found so far to get this (as well as lots of points towards armor upgrades) is Chapter 1 Stage 4, where you start off in front of 3 walker units inside of a trench.
Clear out all the bad guys, and once you get the objective to blow open the wall to the tunnel, go ahead and do so.
Here's the important part!!!
The tunnel is filled with ants and ant hills. Simply do not blow up the ant hills and the ants will keep respawning. This is a great area because it's very close-quarter combat, and the ants die fairly easy with the right weapon.
My personal strategy was to place a turret about halfway down the tunnel, and then run back to the tunnel entrance and fire explosives.
Again, this method is also great to obtain armor upgrades... just be sure to finish the mission so that your points count! Each ant kill grants you 45 points on Inferno, and they die pretty quickly.