Pinnacle of its Kind Quest Can't defeat The Formless One

I'm trying to defeat the dragon boss at the end of Pinnacle of its Kind quest, but when the dragon's health gets down about halfway, it suddenly stops taking damage. I've been at it for a good hour, and nothing; it just sits there attacking and its health is unaffected, even though damage numbers keep popping up. I've tried re-installing and re-loading but that hasn't worked. Game is unmodded.