Dragon Hunting is actually one of the worse maps to do for unit exp. Before the desert maps came i was running Flame Dragon to get a good mix of everything (DH was basically the lazy man's way out when we did not have commission sortie).

But yes, Oasis is a good map to farm until you can manage Base Scramble 2.

As for Bernice, that is your own choice. She is certainly a better tank than Leanne but most of the time your second tank does not need to be as good as your main tank. The CR could be useful. It depends on yourself. I am going with a "every unit is unique" rule, which means i do not allow double units in my empire. Thus double Bernice would be a no for me. If you have no qualms about that, go ahead and level that second Bernice.

As for HAT units i guess you have them all CC'd and at a decent level already. For healer, Iris is the best to continue leveling first and archer-wise Spica will prove more useful but Bashira is not to be forgotten either. I guess i don't need to mention heavy.