I didn't have many issues with her... 100% block shield (Drangleic) means the only attack you need to worry about is the head based ones, which are all avoidable. Try to fight her with yourself on the edge of the circle, facing in, because you don't want her to jump back into the poison and heal.
Oh and burn the windmill. Someone said that lowers the poison pool in her room, I can't confirm because I did it before I fought her.
Side note, I just found a black knight halberd in shaded woods before that boss and it's pretty crazy; 213/0/90, 12 str 26 dex req. Requires using a Yore branch to move a frozen lion knight, who also dropped a Lion Greataxe.
Also, killed the Scorpion guy after talking to him for the yore branch reward. He dropped a +hp/sta/equip load ring.