As Nyx said, I'd do Sez first absolutely. He's a beast in Arena, really good on Wyveren (Farming speed suits will be important) and he farms exp and catalysts for fodder like a beast, only a couple units better than him. But he farms the demons that drop twisted fangs needed by Lorina for her skill level ups especially well--the new side adventure's final stages are great for this, too, and a 60 Sez that's decently geared can solo them with another decent carry and the rest fodders (Four units that drop twisted fangs, level 49--so tons of exp). If your close, now is an especially good time for a farming Sez. (Seriously, the second to last level in Queen Kise's stage is excellent for farming if you need these catalysts and have a good water farmer--all fire demons. But you miss out on the chests from the winter miracle story =-/, so probably best to only farm until everything is bought.)
Also, if you're halfway to Lorina's change--you'll most likely be able to do another 6 star before you swap her, the urgent mission gate pretty much ensures you need to do a shit ton of grinding, which will net you a ton of exp.
