As the trophy name sort of implies, the Gnorc has to first "moon" you (if you're not familiar with this term, look it up on Urban Dictionary), after which you have to shoot him with the cannon.
You can "aim" the cannon by pushing it against it from either side to cause it to pivot, and then press the "fire breath" button to ignite the cannon. The gnorc doesn't have to be mooning you at the exact time you ignite, but you have to at least shoot it shortly after it moons you.