Saenai Ossan, Jitsu wa Saikyou

Satsuki, the daughter of a former comrade, appears before Renji, who was leading a slovenly daily life. "Teach me how to conquer Dungeons." Currently, Dungeon streaming has become a trend among young people. High school girl Satsuki is no exception. Renji was once the strongest conqueror in the country, but ten years have passed, and he has been completely forgotten by society, now just a dull old man. Dungeons, after all this time... Though that's what he was thinking, he couldn't just brush her off and ended up diving into a Dungeon... The dull old man who was only strong ten years ago is actually the strongest even today! A suddenly appeared out-of-date dull old man stirs up a storm in the Dungeon streaming world!!

(Source: Micro Magazine-sha, translated)