I've Been Killing Slimes for 300 Years

I've 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) is a light novel series by Kisetsu Morita with illustrations by Benio. It has been licensed for English release by Yen Press.

Plot
In her previous life as an office worker, Azusa died from overworking. So when she finds herself reborn as an undying witch in a new world, she resolves to take it easy. So she ekes out a living by hunting the easiest prey possible- slimes. But after centuries of doing this simple task, she's ended up insanely strong!