There used to be a Labyrinth exploit that could get you to millions of exp and 1000 ap per run, but unfortunately that's been fixed already. What I suggest is just like what the person above stated. Stick to raid bosses. I think the order goes something like this if you want to level up at a decent pace:
1. Dethrone the White Tyrant
2. White Tyrant's Challenge or whatever it's called.
3. Shadowed by Darkness.
4. Hidden
5. Weeping Queen
Well I really don't know after that because I'm not sure about how good the new missions are in terms of exp and ap. Just do all of the above at the hardest with Season of Macha.