Neighborhorde

Pretty self-explanatory, however you may be wondering if the AI killing enemies will count towards this. Good news, they do. So, once you've completed all other achievements and this is likely the last one you got, no sweat.
Put the mode on easy, load up netflix on your laptop or computer, and just remember to hold A after you stop feeling controller vibrations, or when you see the round is over. I averaged about 600 kills per playthrough without even pressing the trigger once. Enjoy!