From Old Country Bumpkin To Master Swordsman Volume 2

Old swordmaster Beryl Gardinant has become a special instructor for the order of knights. His former disciple Serena, now a black rank adventurer, asks him to help her go on a dungeon attack. Though he has almost no dungeon-attacking experience, Beryl accepts her request. But when representatives from the adventurer's guild doubt his strength, he somehow ends up dueling Serena...?!