This obviously counts through all of your playthroughs and will take you about 5-7 total playthroughs to get this many kills. You could potentially find a spot with a lot of enemies and just keep killing them, and reloading the checkpoint (it will save your kills regardless of hitting a checkpoint or not). You can check how many kills you have by going into the start menu and going to statistics then looking for "Enemies Killed".
*I take no credit for this solution, I give full credit to IIII epic III for the solution*http://www.xbox360achievements.org/forum/showthread.php?t=20...
This is my own findings:
I know off one spot you can farm kills. Near the end of chapter 2 after saving Murray, you will end up been lowered by a crane (done by Murray climbing to the controls) with you and your partner standing back 2 back on a concrete slab attached to the crane cable. I got about 30 kills here, could have got more except for my AI partner killing a few. Once this sequence is over ie your at the bottom, just restart the checkpoint which is just before Murray climbs the crane. Rinse and repeat.
As pointed out by fellow TA'er *tragic1978* you can plug in a second controller (splitscreen co-op mode) so that you get all the kills as the AI control will be overwritten.
Another good place to farm kills is on the "Sink It' mission on Chapter 6. Once the ship blows up you will get a checkpoint save, to your right is a staircase which you go up. Ahead should be a wired fence with a wired gate shut. Put the AI on hold position and just aim at that gate and loads of enemies will run out in your sights. Some will sneak left but you should be able to get them inbetween and towards end as the enemy numbers dry up. A shotgun heavy will come too but I always got him before he kicked the gate open. I got about 50 kills here. Rinse and repeat, just restart the checkpoint once all enemies are dead and gone as it will take you back to just after the ship explodes. In other words don't proceed through the next revolving door as that will trigger a checkpoint.