The World's Strongest Rearguard: Labyrinth Country's Novice Seeker, Vol. 1 (manga)

Workaholic Atobe Arihito has died and been reborn in "Labyrinth Country" where he's now a dungeon hunter. He's running out of time to join a group, and when someone finally approaches to offer him a job it's...his ex-boss!? Here starts the adventure of the strongest supporter!