: After defeating the Demon King and achieving max level, the protagonist becomes overconfident and dismissive of the remaining demon forces.
It sounds like you're referring to ( Yuusha-chan no Bouken wa Owatteshimatta 1 new ), which is a doujinshi (fan manga) by the circle Rotte no Hon-ya (ろっての本屋). yuushachan no bouken wa owatteshimatta 1 new