Grabbed 3 friends (Spriest, Moonkin, hunter) and hit up Naxx 10 last night for some fun and gold. While they were flying, I cleared spider and construct trash (summon on CD); then we cleared Spider quarter and moved on to Plague. After destroying the first boss we moved on to Heigan. The three of them remained on the platform while I danced with him. When it was all said and done, I got my achievement.
Now, they all thought it was pretty funny that I didn't have it given how many times we've been in Naxx both during LK and after. Fact is, as I pointed out to them, I had to do it solo because no matter what, my luck always came to some schlub dying randomly, even at 80+ to the geisers.
Anybody got a similar story on an achievement you just couldn't get because you have to (generally) rely on others?
Oh, our druid also brought up that she missed out on 'the undying' because another healer died to spiderlings when their last boss was the end of the spider quarter....