I've Been Killing Slimes for 300 Years and Maxed Out My Level

is a Japanese light novel series written by Kisetsu Morita and illustrated by Benio. It was serialized online since 2016 on the user-generated novel publishing website Shōsetsuka ni Narō and, in the following year, it was acquired by SB Creative, who published the first light novel volume in 2017. The light novel has been licensed in North America by Yen Press, who published the first volume on April 2018. It has received a manga adaptation by Yūsuke Shiba in the same year as its debut of the light novel in Square Enix's Gangan Online website, also licensed by Yen Press. An anime television series adaptation by Revoroot aired for 12 episodes from April 10, 2021 to June 26, 2021. The series has also spawned various spin-off novels.

Plot
Ordinary office lady Azusa Aizawa died due to overwork. For that reason, when she finds herself reborn as an immortal witch with the appearance of a 17-year-old, she decides to live an easy-going life. Her main source of income is collecting magic stones that drop off the slimes that live in the nearby forest and selling them at the nearby village's guild. After doing this for the last 300 years, she's unknowingly become a level 99 witch, the most powerful of all.

Official

 * Official light novel website (in Japanese)
 * Official manga website (in Japanese)
 * Official anime website (in Japanese)

Other Sources

 * Light novel
 * Manga
 * Anime
 * Manga
 * Anime
 * Anime
 * Anime
 * Anime