Well, after almost 2 dozen tries I gave up. Was chatting with some FC folks and they ended up taking me through it. Much the same thing happened, I did avoid the smashes but even if clear and standing right by a teammate, I got hit for three quarters of my health and they took zero damage. Died 3-4 times. The next 2 bosses (which I had never seen before) I got through with I think only one death. Hopefully this is an outlier...