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

Alina Clover thought being a receptionist for the Adventurers Guild would be her ticket to the good life…but her dream gig turns into an overtime nightmare whenever adventurers get stuck clearing a dungeon. To save herself from paperwork, Alina turns to the simplest solution at hand -- beating down the monsters herself! A mysterious adventurer showing up out of nowhere, soloing the boss, then disappearing without taking credit is bound to create rumors, and Alina's treasured job forbids side hustles. So there might be trouble when the leader of the Silver Sword party discovers her secret…!