This is done on the very first level. You will need to draw gunpowder to the barrels behind the building. Destroying the building will also get you the 3 pigs for this level, which are needed for the Desert Dominator achievement.
You can follow the gunpowder pattern in this picture. Just connect the one tail to the fire.
EDIT: According to the comments below, this may not pop on your screen when it unlocks. If it doesn't check xbox.com to see if it unlocked for you. Thanks Everybody.