From Old Country Bumpkin To Master Swordsman (Manga) Vol 02

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...?! Yen Press
(W) Shigeru Sagazaki, Tetsuhiro Nabeshima (A) Kazuki Sato