01 Jan 2009 09 Nov 2013
There is an easy way to get this:
Start up a campaign of No Mercy on the final chapter on EXPERT. Start by killing off your AI teammates and then grab a pipe bomb. Leave the safe room run up the stairs and into the hallway. Immediately throw a pipe bomb down the hall and then run back to the safe room. After the pipe bomb blows up run back up the stairs into the hallway. Next run back to the safe room and close the door behind you.
A boomer, smoker, and hunter should spawn. Stand a good 3 or 4 steps back from the door and rubberband your controller's left trigger down. The boomer will constantly puke on you from the stairs summoning a horde of zombies every 45 seconds. you will melee them all to death.
I rubberbanded my controller and left it on all night and got around 30k kills in one night. Did it again the 2nd night and got the achievement. Send me a msg if you have any questions :D Happy Hunting!
Note: Be sure to go all the way back to main menu before quitting to dashboard. There are reports of people losing kills while quitting to dashboard from the loading screen.
*Still works as of November 2013*