Ok here's what I did. It appears to be the fastest way to get this achievement. I read what peeps above me wrote and i did a mix of all. I'll try to make it clear as possible. Here's the steps:
1.) Play expert on No Mercy Rooftop Finale
2.) Once game starts kill other survivors
3.) Pick up a pipe bomb (and maybe auto shotgun to kill close up)
4.) Go upstairs and throw the pipe bomb down the hall about half way.
5.) Go back to safe room and get the other pipe bomb and repeat step 4.
6.) Run back into the safe room and shut the door
7.) Wait until you see the smoker and hunter that spawn and kill them, but NOT the boomer.
8.) Sit back far enough away from the door so the boomer can't claw you, but can vomit on you.
9.) Face the bars on the door and rubber-band the Left trigger on your controller down so you continuously melee the bars of the door.
10.) Finally just chill. The boomer will vomit on you every 45 seconds or so and a horde of about 15 will show up. Basically by the time you melee the horde to death, the boomer will vomit again bringing more to you. And as long as you don't leave the room the smoker and hunter shouldn't reappear but they have on me before, so just kill them again and they shouldn't come back.
***Be sure to check it periodically because the boomer may move to a bad spot and unable to vomit you or something else may happen. I tried leaving it running all night and get the achievement that way but when I checked it in the mourning i was on the start menu and lost ALL my kills and was back at 10,000. Even though your character is swinging away the game thinks your not playing after so long of repetitive actions. So just move your player around every few hours or so and you wont run into that problem. Plus you may want to quit to main menu after you get a lot of kills so it saves where your at. I got it done from about 10,000 kills in around 16 hours of doing this method if you wondering. You get about a couple thousand every 30 min. If any questions message me and I'll try and help out. Hope this helps yall : )