Dungeons are gaining an extreme EXP boost from this patch and you can run them more than once.

But honestly, if your relying on Thunder, AND somehow can't seem to succeed at FATE farming, then I think you have a case of the BAD. Conjurer and Arcanist are amazingly easy to get golds with in Fates, Bio/Aero is all you need. Tab > Aero > Tab Aero. Gold. Next Fate.

Or... OR!.... Now stay with me here.....

Get a FATE party, and watch your medals go to gold for casting 1 single spell on someone in the party.

I Know... I know.. that sounds like a lot of work for 5-10% of your whole entire level's EXP per 5 minutes.... but... I guess your just going to have to deal with it.