The Tormentor doesn't award the achievement
Spent all day trying to get the Tormentor achievement today. First two times I wasn’t sure if I’d dealt enough damage, but third time I did, so pretty sure this is bugged. Got neither achievement nor mastery point.
Next time, be sure to be in a party — the amount of damage you need to do for scaled-up bosses is (apparently) a lot more than we think. I don’t think the Tormentor is ‘bugged’ any worse than many others — I think it’s more that it’s too hard to tell just what we have to do to get credit.
Alternatively, try to be on siege, especially burning oil (assuming that people are able to lead the boss close to the fort).
John Smith: “you should kill monsters, because killing monsters is awesome.”