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

The story

After dying of overwork in the real world, I’m reincarnated as an immortal witch, and I spend 300 years enjoying a relaxing life. At some point, though, I end up at level 99! All those years spent killing slimes to make the money to pay the bills gave me a ton of experience points. Rumors of the level 99 witch spread, and soon I’m up to my ears in curious adventurers, duelist dragons, and even a monster girl calling me her mom! I’ve never been on an adventure, but I’m the strongest in the world… What’s going to happen to my relaxing life?!