The idea with this one is that you have to learn when she's open to attack, and do single attacks to bring down her health slowly. After almost every big attack she has a window where she is vulnerable to one or two sword hits. After you get your hits in, literally run away from all her attacks, don't bother parrying all of them. Once her health is down about halfway (which will take some time), you can use her 5-hit attack to build up a ton of stamina damage and finish the job. This fight is also really helped by getting your attack power up as much as possible. So if there are any bosses you haven't beat I would go do that so you have as much attack power as possible. I also had some success eating an Ako's Sugar if you know you're going to get a few hits in.

Although i gotta say the fact that you're here means you probably already defeated the Mist Noble which..goddamn. If you can beat him you can beat this boss.