Exposed Weaknesses (Hidden Arcana) also bugged?

[Dschromm.2946]

We tried to get this achievement in a 2-men-party during our replay of  "Hidden Arcana". We managed to get the facet from 75% health to 0% health in approx. 5 seconds and didn't get the achievement. The qualification for this achievement persisted throughout the rest of the story instance until the very end.

[De Dominator.9834]

Necroing this thread, because I am having the same issue.

Did you ever find a solution? Because I have tried everything the wiki suggests, and just to go over all that for the sake of clarity.
 

Quote

Exposed Weaknesses: Reduce the health of the facet of darkness from 75% to 0 in less than 15 seconds.

Occasionally the "Exposed Weaknesses" achievement will bug and players need to get the facet from 100% to 0% in the given time interval; this is performed by destroying all crystals at once instead of pre-emptively destroying two. Sometimes the achievement will also require that the player(s) hit the Facet itself at some point during the fight; just destroying the Vortex Crystals without hitting the boss may not be enough to complete the achievement.

 

Tried everything suggested, afflicted 2 of the crystals with fragility then destroyed them to get it to 75% and then destroyed the rest within 15 seconds to get it to to 0 in less than 15 seconds. I went and took it from 100 to 0 in one go, I tried attacking it myself then doing both of the aforementioned things. None of which earnt the achievement. I replayed it and tried to complete it again twice but it still didnt work.

Havnt found anyone talk about a solution so hoping to hear if someone has figured it out, or if it is just broken.