I got this one first try on Challenge 3, the same one best used for cracking out
and all the stackables that go with it.
For this achievement I only needed to set it to Normal difficulty (I didn't try Easy so not sure if enough zombies will spawn). Once you wake up exit the spawn and grab your weapon of choice and some ammo, run up to the top of the map and round the back of the spawn shack to grab some fresh meat.
By the time I'd got the meat and continued running round and was back at the door of the spawn there were quite a few zombies milling about. After kiting them a little while longer I ended up with two boomers as well, once they're there and there looks to be at least 20 zombies, chuck down the meat. I then ran round to the back of the group (as the boomers walk slower) and once they were all fairly close killed the first boomer and switched to the second boomer, with both these explosions you should wipe out the group of 20+, do keep shooting though to finish off any stragglers, just in case.