Killed him 3 times today, even recorded it, made a ticket, and got told this.
So...we're out of luck...
Greetings,
The Elder Scrolls: Online Customer Support does not have the ability to give or restore in-game Achievements.
Please be aware that most of the achievements linked to a kill or a certain dungeon can be unlocked by repeating the task again.
In order for the achievement to unlock for boss objectives, you'll need to contribute significant damage to the boss (and sometimes boss "monsters" in the area), or significant healing to warriors engaged in the same fight.
If you fail to contribute enough damage to the boss or not enough to the additional mobs, you will not receive credit for the Point of Interest and you will not receive the achievement. Your best bet is to attempt the boss again and ensure you're damaging each monster in the fight.
If you believe there is a bug with an achievement causing your progress to not be updated, please submit a bug report by using the /bug command (if playing on PC). On console, a bug can be reported by creating a new ticket and choosing "Submit Feedback" under "Issue Category".
We strive to provide excellent customer service to players in need of assistance. If you would like to provide feedback on your customer service experience, please fill out the brief survey that you will be receiving shortly. Your feedback can help us improve your experience!
Kind regards,
The Elder Scrolls Online Team
https://www.youtube.com/watch?v=yqYS4hdZN98
And
https://www.youtube.com/watch?v=TIIO6vmPSQM
Took me killing him 4 times to get credit. Talked to someone who did 18 times still no credit. The one I can't get is the Lady Malygda. She will not spawn for me to kill. Been trying to find her for 3 days now no luck.
Be safe and have fun:)