There is one worrisome possible way that Bell could win but it would be a really bleak ending. If one of my earlier theories about the dungeon's true nature is correct. The theory being that the dungeon is actual part of the primordial goddess Tiamat who in Mesopotamian mythology was kill by the other gods and her body was cut in half. Half was used to create the heavens and the other half was used to create the earth.
In the mythology, she created monsters after her partner was killed. Sometimes she is depicted as a dragon or a sea serpent and other times as a woman.
If my theory from years back is correct and the dungeon is a part of Tiamat's body that holds her consciousness and power. Then they might be a way for them to win but it will require a heroic sacrifice.
If the corrupted spirits are in some ways an off shot of Tiamat's mind then it may be possible to bargain with her but the price would be a steep one. We have been hearing up until now how pure Bell's soul is. What if her condition for ending all of this requires Bell to offer up his very soul to her for her to use to decide the fate of the world on.
If she sees by looking at his soul that the world is worth preserving then the OEBD will cease its attack. But if it is not enough than she will have it devastate Orario and then the rest of the world.
In this way, Bell would be a hero that saved humanity. Not through violence but a noble and selfless sacrifice. He, who is only 14 years old, would willing give up everything he has to save everyone.