Regarding the enemies always targeting Nina - I found this frustrating until the point that I got an Earth Crystal drop (which can raise defense significantly). When I equip that with her starting armor there seems to be a bug that she can dodge far more than usual and the damage she takes is extremely low, to the point that she can tank most battles. The exact same loadouts but with different armor equipped, she will take many multiples of damage. If I want to use Nina, I pretty much have to take the strategy or she gets creamed immediately.
Looking forward to the 5.0 drop!