To get this you must have:
(a) a two handed weapon, such as a sledgehammer and
(b) the Ground Pound (under the Power skill tree) skill
So what you do is:
1) Go find a bunch of infected clustering around a bus. I went to Infamy Bridge over on the far right of the map because it's crawling with infected and you have to get to the bridge for this:
2) Hop on top of the bus
3) Throw down a firecracker to get the infected to bunch up.
4) When they are clustered, face the infected,
5) Press
to jump and as you are mid way down hold
. You'll know you're starting a ground pound because motion slows down.
6) Now turn around and go back to step 2
Here's a video...
http://1drv.ms/15JkUdE