Can’t go on with the story

I’m level 66, reputation 11 and battle grade 22. I’m stuck at hidden chapter 3, second 65th level battle, I can’t even go on in difficult mode because in chapter 18 at the one before the last battle at level 70, it says that I have to reach level 71. Inversion mode doesn’t give exp, so I don’t know how to reach the level I need to continue. Gates give too few exp. What can I do?