Expose weakness redux achievement.
You’re intended to use the vortex crystals and not DPS him down over an hour. Just don’t give him boons such as might. The achievement will not register if you DPS it down.
Apparently there was a bug at the time we were doing this that said that the shatter buff was registering as a boon. Also my friend who was doing it at the time got the achievement.
That particular achievement is likely player-based just like how the mine the dead bodies achievement is party-based. That’s probably why your friend got it and not you.
This issue is still present. I just did this achievement properly twice. All my teammates got it. I tried staying away completely for the second round with pet hidden. After the boss died, I still had the achievement marker effect on me, and when I tried for the second time, I had two stacks of it.