Im getting spanked even with healing , shes way too over powered. i always get her to a pinch of health then she kills senya and then im left with no companion , and i die.

I've been stuck on this boss named Voice of the emperor for 3 hours spamming buttons trying to get his health down, but everything i do is trash and he just keeps parrying and healing himself so it's basically impossible to beat him. I just wanted some tips on how to beat him if anyone knows.