Slime Taoshite 300-nen, Shiranai Uchi ni Level MAX ni Nattemashita (Novel)

From On: 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 vowed 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 power. How will she maintain her low-key life now?