No Doubt

Blessings and Torments are fixed each day for a particular wing of Torghast and reset when daily quests do. So if you want this achievement, you'll need to keep checking back every day for when the Torment "Backup" is active.

Once you find the proper torment, this achievement can be very easy or very hard depending on your class and anima powers. The final boss will come with an add that doesn't have a lot of health that will cast slows and dots on you. You must kill the boss without killing the add. I failed the achievement the first time by fearing the add as a warlock, and that apparently counted as interrupting its casts.

So no CC and watch your cleave damage. Go in with a group and whittle down the boss if you're having troubles doing this solo.

No need to tryhard if the boss doesn't regen health and you can die as many times as you want. Just singletarget the boss and ignore the add. Easy peasy lemon squeezy I'm such a genius.

If you are confused like I was, by "boss" the achievement refers to the very last boss of the tower (layer 5), not a floor boss like I thought.