In a 2e campaign we were in an underground complex and looked inside a large room to see this thing at the far end (about 200' away). It was standing in a pile of bodies and dropped the leg it was chewing when it saw us, and charged the door. The DM told us we had one round to prepare before this 20' tall, 6-armed monstrosity (an aberration creature our DM made up) hit the doors. I shut the door and asked how tall the ceiling was here. The DM said, "Twenty feet, why?" *Insert evil smile here* I said, "I take something out of my belt pouch and sit on the ground about 10 feet outside the door," then I said, "Everyone stand about 20 feet back from the door and when this thing comes out just shake your weapons at it and scream." Everyone just stared for a second, including the DM, so I said, "Trust me."
Well, the BBEG bursts through the doors and steps over the object I laid down. The DM says, "What do you do?" The party did as they were instructed, as the creature stopped and stared at the apparently crazed adventurers. Then I said the command word for my
Daern's Instant Fortress. It did 10d10 damage, crushing it against the 20' high ceiling. Not that it needed it after that, but I did it 4 more times for fun. Twenty foot tall creature vs. a 20' adamantine tower and solid bedrock. Ouch. The DM nearly cried. I suggested "Pancake Demon" as a name for the monster. He was not amused.