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

After living a painful life as an office worker, Azusa ended her short life by dying from overworking. So when she found herself reincarnated as an undying, unaging witch in a new world, she vows to spend her days stress-free and as pleasantly as possible. She ekes out a living by hunting down the easiest targets—the slimes! But after centuries of doing this simple job, she's ended up with insane powers...how will she maintain her low key life now?!

(Source: Yen Press)