NPCs actually did just that, setting up a barrier on the last underground space layer that even an emperor-level boss couldn't break through, under the assumption that Bone Demons wouldn't be able to break such a barrier, perhaps ever.
Even Zhou Tian could be said to have lost face against that barrier. He had planned to shatter the barrier with sheer strength, but to his surprise, it remained unaffected by his assault. Seeing the Bone Demons witness that performance, Zhou Tian naturally felt rather embarrassed.
For a typical emperor-level boss, given the barrier's capacity to withstand such force, it probably really had no way to deal with it. Faced with such a barrier, I reckon that nine out of ten monsters would choose to give up.
