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

Aizawa Azusa, a corporate drone who died of exhaustion from overwork in her previous life, is reincarnated in another world, where she decides to live out a slow and uneventful life forever. But then, 300 years later, she discovers that the experience she earned by defeating slimes every day just to keep up her easy lifestyle has brought her to level 99 and made her the strongest in the world. The rumors begin to spread, and soon trouble comes knocking...

After becoming the strongest in the world before she even knew it, Azusa entered into a new lifestyle with her first pupil, Laika the dragon, and discovered fun and enjoyment in it that she hadn't known before. But then, one day, a young girl calling Azusa her "mama" shows up and proclaims that her younger sister wants to kill Azusa. But Azusa has no memory of giving birth to these two girls, or of becoming a mother at all...

Just as peace is beginning to reign in Azusa's home, a young elf named Halkara shows up pleading for her help. Halkara says that an energy drink she made angered a terrifying high demon, Beelzebub, who is now out to kill her. Azusa would prefer to avoid anything that might ruin her slow life, but she can't just leave the poor girl to die, so she reluctantly agrees to shelter her...