I've been trying for a while to get this achievement using a two controller versus method without clearing my cache or removing the patch. In the end, I managed it like this:
* Load up versus on the dark carnival, rock concert finale. The gamertag who needs this achievement must be the infected, the other controller can be any survivor but it's recommended you pick your character so you can tell who's who - for the sake of this example, choose Nick.
* Use Nick to quickly kill one of the other survivors.
* Use Nick to open the safehouse door and keep killing the computer controlled infected until your main gamertag is due to spawn as a charger. Defend the safe room, do not leave it.
* Once you're due to spawn as a charger, move to the back of the saferoom, around the corner but don't spawn yet. If you see a message saying 'restricted area', send Nick down the hall and bring him back once the message is gone - usually as far as the restrooms is enough.
* When it's quiet, use Nick to kill the survivor with the least health and heal up the other survivor - for this example, assume Ellis is now the only computer controlled survivor.
* Now, assuming you're not under direct attack, spawn the charger and move the Nick to the back of the safe room so that Ellis follows. Get ready to charge *Ellis* with the charger - you won't get much time
* Use Nick to guard the charger and Ellis from any other attacks whilst the smashing is in progress.
* You should find you get the achievement even if Nick dies after a while as Ellis continues to suffer through the score screens.
Have patience and try again if you don't get it the first time. I'm afraid those 15 seconds seem like a very long time once things are set in motion.