Load up Goat City Bay. From the large skyscraper hotel, walk a bit towards the amusement park and you should notice a sewer entrance. Once you enter it, the achievement will unlock.
Once you're here, you might as well get another cheevo at the same time.
Keep going a little forward and enter the opening on the left. Now just fight the grown men dressed in green (=turtles) by clicking the left mouse button and after you've headbutted them a couple of times the achievement should unlock. Video below, credit to Maka91.