This is unlocked on the 2nd World on the home level. Take out the Gnorcs manning the cannons, and go near the other Gnorcs to have them run into their tents. Once inside the tent, burn it down but don’t harm the enemy. The Gnorc will then proceed to flash his arse and point at you whilst laughing.
When he is doing this aim a cannon and use flame to set it off. The cannon ball will auto-aim if aimed close enough to a Gnorc and the achievement will pop.