After you have found all the DA members, you will find on your map that 4 of the Members have new quests for you to complete. These quests include setting of Swamp bombs, messing up the clock and messing with the PA system. There is a Swamp Bomb in each of the courtyards, so make sure you visit each one to set them off. There are 6 PA systems in total that you must corrupt. Each time you complete one, go back to your map and find the next area that the Pavarti sisters are located. Once you complete those, you will take control of Fred and George on brooms, and have to light off a group of fireworks in the Grand Staircase. Finish that, and achievement unlocked.