Feral Fury (EP)

This one is pretty self explanatory but in case its not obvious:

Lizards sometime spawn when you return to rooms. you simply run/roll over them to squash them. They appear randomly and you will likely get this achievement during normal gameplay.

I'm not sure if it was the luck of the draw or not, but the secret level (commonly found behind a false wall on 2-1) was literally crawling with them. Just rolling in and out of two different rooms, I was able to get 6 in a matter of minutes. In my playing through on the other normal levels they seem much more rare.