Wow. What a battle. It took me less than a minute to reduce her health to about 10% then for about twenty minutes it remained at 10%.
In her bubble nothing affects her. I don't know how I eventually defeated her. Bombs, signs, sword did not affect her.
You have to attach her when she is out her bubble but before I could reach her she was off again.
Syanna eventually arrived and there was a cut scene so I am not sure whether she finished that witch off or not.
I was on the verge of giving up, assuming it to be a bug.

She's completely immune to everything while her bubble is up. The key is to wait until she rushes at you, because her bubble comes down. If you use Aard or a crossbow as the bubble comes down she drops to the ground and is vulnerable. It's a matter of timing and patience.

I was using aard and yes, you do need patience, plenty of patience.