Osananajimi to no Yakusoku wo Hatasu Tame ni Hyakunen Shugyo Shita Rouyuusha, Nokori no Jumyou de Maou wo Utsu

Berg, whose parents were killed by demons, sets out on a journey to defeat the Demon King. On the night his village was attacked, he promised his childhood friend that he would slay the Demon King. To fulfill that promise, he secluded himself in the mountains and trained relentlessly. One hundred years later, now 110 years old and armed with overwhelmingly powerful punches, Berg finally descends from the mountain. On his way to the Demon King, he encounters Eruruna, an elven swordswoman fighting against the demons...