This outcome feels a bit deus ex machina-ish. You could say it was kinda foreshadowed when Laios talked about how demon inherited our world desires, but it's still too convenient that he just wrote down cheat ability and that's it. I like the resolution itself: very clever and in line with manga themes, but the way it was introduced into the story was a bit sloppy. So Laious could even write something like "monster who can eliminate the demon in instant" or "monster who can seal demon dimension" and that's all?
IMHO It would be better if his strongest monster design initially had some quality like "can eat everything that exist in this world" or something like this and Laios was able to utilize this loophole instead of outright shove in some OP ability in the last moment.
But still, the pacing, the art, these scenes with Laios and Lion are absolutely brilliant!