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

After dying from overwork, a woman reincarnates as an immortal witch in a peaceful fantasy world and spends 300 years living slowly and killing only slimes. She wakes up one day to discover she has inadvertently maxed her level and become the most powerful being around. An ever-growing family of unusual companions joins her relaxed and cozy life.