What's better to focus on, EXP levels or class levels?

I am level 17, and at level 4 for most classes. I noticed some time ago (around level 14 or so) that the enemies were scaling and that switching to class leveling seemed to help (was class level 2 the time), but lately things have been getting REALLY hard. I'm having a tough time with the current event, and I can barely make it through dungeons that I used to breeze through. I'm obviously behind the curve but I don't know where the curve IS. As far as progression, I'm at the final battle with Wizpig on top of the mountain but haven't seriously tackled it after getting trounced the first time.