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...

Laika's older sister Leila is getting married, so Azusa and her friends set off to visit Mount Rokko, the home of the Red Dragons. The mountain is bustling with the many Red Dragons who have gathered for the ceremony. Azusa congratulates Leila and the ceremony carries on in peace... until the Blue Dragons, a tribe with a long-standing grudge against the Red Dragons, attack in full force!

Halkara has built a new Nutri-Spirit factory in a nearby town and is now regularly going there from the cottage in the highlands, but she comes home exhausted every evening. It seems rumors have spread saying there's a ghost showing up at the factory, so she's had to do all the work alone since all the locals are scared to go there. Azusa doesn't deal well with ghosts at all, but she can't bear to let Halkara work herself to exhaustion, so she summons Beelzebub to help and heads to the factory late at night.

Azusa receives an invitation from Beelzebub to attend a ceremony in the land of the demons. It seems she earned a favorable reputation with the demons after stopping the conflict between the two Dragon clans in Laika's homeland. Azusa invites the whole family along for this high honor, but there's one problem: Rosalie, the ghost, can't wear a dress. But Rosalie is part of the family, and Azusa wants to help her dress up like the rest of them, so she uses the power of her maxed level to create a magic spell...