Ok, this one is a little confusing because the hidden graveyard is hard to find. Basically It is almost impossible to describe where to go so I will do the best that I can. First when you start out head all the way right until you see a wall right in front of you or until you can't go right straight any longer. Then head left and eventually you will find Larry's truck. From Here head south and you should see a cemetery on the right with like an orange light in it. What you will need to do is destroy that graveyard's headstones and a hidden monster fest will appear. Beat this hidden monster fest and the achievement will unlock. If you don't know which graveyard it is just destroy the one to the left as well.