Inside the cave, Amon never stopped moving to avoid the deadly dark beam from Heaven. He stayed inside the cavern because it had a decent enough space to move around. If he tried to exit the cave, he would have to go through a narrow tunnel. With how often the death beam hit from above, he doubted he could make it to the exit. He couldn't punch through the ceiling either. This cave was too deep. It would only expose him to the beam.
So he stayed inside and focused on his danger sense, waiting for the death beam to descend. Thankfully, the beams were narrow, like blades. The scars they caused to the cave's ceiling were thin slits. They were not destructive enough to collapse the cave's ceiling. If that happened, he would be buried. Without being able to move, he would be done for.
