>>713573687
She's not. you just have to deal a shit ton of damage and not just tank/block her or she will heal too much.
Another thing is people using slow tanky summons that just feed her instead of doing what summons usually do (taking aggro off the boss for free attacks).
By myself when the game first came out I was able to kill her after 2-3 tries but later when I was trying to help my dumbass friends they kept feeding her free hits.
She's also EXTREMELY easy to stun with colossal weapons.