Been playing dark souls 3 all day, I know how it feels...fucking dancer of the boreal valley -.-
Well, that makes two of us. Been working towards that wonderful boss Nameless King, personally, with some good old fashioned pyromancy.
Don't get greedy, get a hit or two in, and watch for the counter attack. Dancer especially during her first phase, since she has a mean grab attack. Second phase, she tends to spin a lot more, so timing the rolls is key. Blocking is kind of pointless, three different damage types coming from her (Physical, Dark, Fire). If all else fails, find a summon and use them as bait.
All right. I'm gonna keep it somewhat vague, so feel free to add in what details you'd like.
