Yeah they are a pain, would have gotten 3 stars if it weren't for them and I had the same problem with them shredding Leanne (she has higher defense then Bernice right now).
I am wondering if a class changed angel can handle them before Nurarihyon comes out what with blocking three and attacking two units simultaneously. Then again my best angel is Erun and her skill is only level 2 so she might not have enough time.
I placed Ertel at the top right, to the left of the ranged slot, she was able to offer some temporary healing before I pulled her out because of those weasels. I am confident a priest armor is useful here since ranged slots are at a bit of a premium and not in the best positions.