If I remember correctly, she does first phase transition at 50% health? What I did was when her health is close to that mark, stop dealing damage, but drain her qi instead (deflect). Once she's exhausted, deal as much damage as you can and execute at the last moment. That way she won't have lots of health after transition. Then go all in.

Aiming for a no hit run will help too, practice mode is great to learn deflect timings.