Everything is described in this youtube video, you can read the description:

https://www.youtube.com/watch?v=6ZnWj839mwg

The gist of it is that you HAVE TO PLAY extremely safe, run away after every hit. That way you are always far enough away when she starts the air slash.

FromSoft should add a delay before she starts the slash so you can always run away, but until they do, you can either follow the video or:

Block the first set of slashes with a greatshield, roll the other 2 as in the video.