Slime Taoshite 300-nen, Shiranai Uchi ni Level Max ni Nattemashita  

Ive Been Killing Slimes for 300 Years, スライム倒して300年、知らないうちにレベルMAXになってました

In this story, a woman named Azusa Aizawa dies suddenly due to overwork. She is then given the chance to reincarnate as an immortal witch in a new world where she spends her days killing slimes for money. However, even with minimal experience points, she eventually reaches the maximum level after hundreds of years. Fearing that her abilities will attract work and force her back to her old life, she decides to hide her strength. Despite her efforts, tales of her max level "Witch of the Plateau" spread across the land, attracting both friends and foes looking for her. Eventually, she befriends a dragon named Laika who challenges her to a strength test. Though she defeats Laika, problems arise as more people become aware of her abilities and seek her out.

Slime Taoshite 300-nen, Shiranai Uchi ni Level Max ni Nattemashita has been published in English as I've Been Killing Slimes for 300 Years and Maxed Out My Level by Yen Press since January 28, 2020.