Jimi na Oji-san, Jitsu wa Eiyuu Deshita. Jikaku ga nai Mama Musou Shitetara, Mei no Dungeon Haishin de Sarasareteta You desu

Keita Sato (41, single) is overworked at his corporate job, dealing with tedious and stressful tasks day in and out. How does he relieve his stress? By sneaking into modern-day "dungeons," using them as a batting cage, and blasting away ferocious monsters! One day, his live-in niece Hikari secretly follows him and livestreams his adventures, only for his unbelievable strength to go crazy viral!! Keita unknowingly captures the attention of not only viewers, but other big-name streamers. So begins the fantasy tale of an ordinary office worker's unwitting rise to fame and glory!
(Source: MANGA Plus)