18+ Gal to Dungeon to Shuukaiokure no Tansaku Eiyuutan

Kaoru Kunou, a former legend known as the "Dungeon Maniac," is now, for certain reasons, a bottom-tier explorer. Though worn out by reality, he continues to chase the fleeting dreams and youth of his past. One day, by chance, he saves S-class gal streamer Shion from a crisis, and she falls for him at first sight. Initially flustered by Shion's overwhelmingly bright and charming advances, Kaoru gradually finds himself healed and drawn to her. Eventually, the two form a temporary mentor-student duo and set out to challenge the depths of the dungeon of their destiny, but!?

(Source: Hobby Japan, translated)