I had a lot of trouble with this one in NG+ until I figured out to stop trying to be behind the boss. Aava is pretty slow on the recovery, and if you're rolling backwards the swipes are easy to dodge.
So what worked for me was just standing in Aavas face to bait an attack, roll it, then whack once or twice with a quick weapon. Took 50/60 durability so watch out for that.
Good healing windows are at the start of the ice soulmass and when aava very first starts running away. (Aava will run away and then usually set up a lunge attack pretty often). You can also wait til you dodge a lunge to heal.
Hope that helps someone