Yuushachan No Bouken Wa Owatteshimatta 3 Best -

If you're asking for the aspect of Volume 3, here are top contenders among fans:

Leo, reverting to his knightly instincts in a school setting, challenges the local delinquents to a duel to "purify the school." Yuusha-chan has to intervene before he gets expelled (erasing his existence). She teaches him that in this world, "protecting the peace" means passing math exams. yuushachan no bouken wa owatteshimatta 3 best