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

Berg sets out to defeat the Demon King after demons kill his parents and attack his village. That night, he promises his childhood friend that he will slay the Demon King. To keep his promise, Berg withdraws to the mountains and trains for a hundred years. At 110 years old, with devastatingly powerful punches, he finally descends and heads for the Demon King. Along the way, he meets Eruruna, an elven swordswoman fighting the demons.