Hey bud - We had the EXACT same issue and ran the dungeon at least 6 times with no accomplishment / achievement. We even went so far as to run the dungeon with NO deaths (not a single one) / no revives and it still did not work. Finally last night we got the achievement to pop.

Here is what we did:
1. All of us (8 total - 2 groups of 4) completely restarted our Xbox One's (turned off console and power cycled)
2. Regrouped into the same groups and entered the dungeon with groups of people within 5 levels of one another (I'm not sure if this makes a difference but I've been told by many, many people that XP doesn't accrue if you play a dungeon that is 5 levels lower than you are via having a group leader whose level is far lower than yours)
3. **This is what I think fixed the actual issue but I included steps 1 & 2 just in case** - We previously were fighting the Sentinel of Rkugamz BEFORE we fought the mini-boss that is directly before him (the mini-boss is a dwarven mechanical robot that spins and goes into a small shell type posture). The mini-boss can be found before turning left and crossing the stream / waterfall to get to the Sentinel of Rkugamz by going to the RIGHT and up a small hill where he will immediately begin attacking you with some other dwarven mechanical enemies. Once you've beaten the mini-boss, THEN go ahead and fight the Sentinel of Rkugamz and beat him. We did make sure that we all survived the fight and for all 4 of us in our group and all 4 in the 2nd group, the achievement popped / was given to us.

I'm not positive if there is anything in the list that makes this happen individually or if it's a combination of all of these things or what but I can say that #3 has also been confirmed to work for a few other members of my guild now who either:
A. Hadn't done the dungeon at all OR
B. Had done the dungeon without getting the achievement

NOTE - I've also been told by a few of the newer members of my guild that they've secured the achievement (after not getting it to work) by playing in a group with someone who already had gotten the achievement. This could be a coincidence but also worth noting.

Good luck and let me know if this helps and solves the issue for you!

I have been having this same issue. After working with the support team they are reviewing this now. Will update when I know more

Yeah been having thr same problem its a real pain in the backside too