I May Be a Guild Receptionist, but I'll Solo Any Boss to Clock Out on Time, Vol. 1 (light novel) — Mato Kousaka

TAKE A HAMMER TO OVERWORK!Alina Clover signed up to be a receptionist for the Adventurers Guildthinking it would be her ticket to the good life -- stable pay, regular hours, and safe workplace conditions. Unfortunately, her dream gig turns into a nightmare of endless paper work and overtime whenever the adventurers she helps coordinate get stuck clearing a dungeon. But people aren't made to grind forever, so Alina sneaks off at night to give the monsters holding things up a beatdown, courtesy of her massive war hammer. Keeping this all under wraps to avoid violating her office's ban on second jobs is difficult, especially when the tales of her exploits start to spread, but Alina somehow manages to keep her identity safe…until the leader of the strongest party in the guild catches her defeating a boss one fateful night! Does Alina have what it takes to stop her life from going topsy-turvy?!