I also had problems with it, but figured it out after a few deaths, what I did is:
After doing Yrden to make it visible, stay in the trap and wait till it attack you first, block and then counter strike 2-3 times, then repeat. Do not hit it until it's out of the circle or it will rush at you in it's ghost form and deal some big damage. The illusions are it's way to replenish it's health so move quick and beat them down. Repeat again with the real one until it goes down for good.
This seems to be working. Thanks. Still haven't beaten her but the fight seems much more doable now.