If you are using summons, id recommend the black knife assassin. He'll some times dodge the dance. Can make her bug back slightly. And his blade causes her healing from hits to be irrelevant. Kinda just a cheap way to win but ay.
I summoned help, only one though as 2 reduces damage. distract him with your summon, then backstab over and over. Ashes summon didn't work too well here. Human summon only. He was tough.
My first 15 min in the dlc. "Wow beautif- did I just get assassinated?: "I am staying away from that monstrosity. I've seen Wicker Man." "First boss let's go- did I just walk into a drive by?"
That sword is probably best for humanoid enemies, you can just spam skill to knock them back and till they get up you're already doing next skill. Easy stunlock basically. Just that he in video didnt dodge arrows first :D