I remember minori gang get from 22 - 35 after the goblin arc.
And
http://mamare.net/character/
up to volume 7 william get to 95. That despite he was occupied with bullying delicious.
Just follow a 9x gang to eat spare exp is more than enough
But after all, this is a fic. who care if she just drink a pot and boom, insta lvl 90
