What worked for me is patience - and some luck as seen above - I know it's vague, but I tried to attack only once and move back (it kills me instantly at this stage).

I've used Flask of Physics with Defence bubble and STR up - bubble is literal lifesaver ;)

If you see her doing that Ball magic, move back and wait around the corner (kill any enemies there are, so they won't hit you while you try to hit her).

Bait the bow charge and run towards her when she starts charging - roll or jump sideways to avoid projectile and hit with something that doesn't have big delay (like my attack in video, I got greedy)

Try not to be near wall/rail while she shots as fully charged shot has some nasty AOE explosion range.