I was playing earlier, and found a somewhat of a solution to the achievement.
Start a single player campaign. You're going to do No Mercy on Expert, on the rooftop finale. Once it loads up, grab a shotgun and kill off your fellow survivors. Expert makes them killable in 2 or 3 hits, so just start on there. You can now boot up the start menu and call a vote to change the difficulty to easy. You may need to change back if you end up messing up, which I found easy to do. Grab a pipebomb, shoot any zombies blocking you to the top of the steps. Chuck the pipebomb down the hallway. It'll kill all those zombies, and should trigger an attack/swarm, along with a Smoker, Hunter and Boomer. Go in the safe room and shut the door. Kill off all the common infected, and the Smoker and Hunter. Now, all you have left is a Boomer who will vomit on you, causing successive infected attacks.
The trick is to have the Boomer either not or barely visible, as shown in this picture:
Just make sure you're just close enough to the ammo to be able to restock without moving and you should be golden.
It may take several tries. Sometimes the boomer comes up all the way the door and tries to smack you. Kill it, and new special infected will spawn eventually. All you do is wait for the swarm to show up, and blast the pump shotgun like four times across the door to wipe them all out. Rinse and repeat many, many times.
I found the majority of my swarms to be 28 common infected, and sometimes 27. It'll still take a long time to get this achievement either way. It's very boring to use the same method for so long, but it can help a little. Hope this helps.
I'm currently on 26,000 kills and bored out of my mind. :P