Hezenkoss Boss Fight

Hezenkoss won't die! I have tried save-game reloads, Changing difficulty to max, and all manner of standard fixes... I put off this quest for a while and just ran around grinding to upgrade all weapons, armor and levels... (Lvl 50 Max).  Watched videos where Undead spawn, but that never happens in my playthrough. Spent literally 20 minutes beating on her.  I have zeroed out her health bar 5 times but she just won't die. What am I missing? Is this a bug?