How can I grind more? (Effectively)

Hi again!! I'm struggling a little bit because I've finally reached Mana Castle as I said the other day, but I'm now level: MC: 29, Maki: 31, Nanjo: 28, Mark: 27 and Brown: 28.

I'm in Mana Castle and I'm literally getting wiped by every enemy in there, they're like level 34 or something, I thought, okay I need to grind, but how and where?

In mikage ruins I only can access B2F and the enemies are low level, and in the city I "can" defeat enemies but they give like 100 or 200 XP

Sorry for my English and I hope you guys can recommend me something, I'm loving the game and I'm up to grind, but I just don't know how :p