Just to heed a warning to fellow Glory hunters with this specific achievement left;
Today I was in a group of 10 people for several hours finding and filling people to get this achievement done. We had wiped several times and eventually had a good enough team where we all managed to execute a smash on the same boss during the same dance.
Our RaidAchievement addons and achievement trackers all told us that we were eligible, as well as us all witnessing and confirming that no one missed it, so we went ahead and killed the boss.
AAAAAAND didn't get the achievement. We were all like that's strange and some people passed it off as we must have screwed something up but some of us weren't so sure so we sent in tickets.
A GM just contacted me and we had a chat, turns out that currently there are raids reporting that the same thing has happened and they did not receive credit despite executing correctly.
The reason why this is bugging is that there seems to be an issue where if you wipe for attempts before hand your final and successful attempt will not be credited correctly and you won't get it. There might also be other aspects but this is all the GMs have noticed as being a trend. The room also bugged for us and adds kept spawning from the walls.
So I gave him all the info from the raid, our wipes and etcetc, then asked what happens now? Do I get credit for the achievement now or do I get credit when they realise it's a known bug? (because who really wants to go through LFG for god knows how many more hours trying to get people to do the achievement)
The response was that even if they figure out its a bug the logs are too hard for them to look at to confirm it (even though I have several screenshots of addon confirmation that we executed it) so I probably will never receive credit and I'll have to spend several more nightmarish hours trying to get this achievement done!
So if you were planning on getting this achievement done, either hold off until they fix what's wrong, or get the best damn group you can and do it on the first attempt (wish you luck with that!).
Otherwise you run the risk of spending hours trying to do it only to have them wasted!