From the Super Mario Wiki
The Gargoyle Roof is an area in Luigi's Mansion: Dark Moon, more specifically in Treacherous Mansion. It is the only area on the third floor, and has no main part in the story as no ghosts ever infest it. It can be accessed from the Ice Age Exhibit.
It is set outdoors and is decorated with gargoyles. The night sky on the background shows a constellation shaped like a star, but three slots are empty. If Luigi uses the Poltergust 5000 to catch shooting stars or take one that a gargoyle holds, he can fire them at the constellation. If Luigi misses, another star will crash land to try again. After completing it, the Pebble diamond gets launched out of the middle of the constellation and lands on the roof.
The Gargoyle Roof can only be accessed in the missions Double Trouble and Terrifying Invasion. During these missions, Luigi must pull the mammoth's trunk, causing it to launch him upwards giving access to the area. In other missions, the mammoth is stuck in the ice cave and cannot toss Luigi.