I was doing LFR on horridon boss. Died, got battle ressed then had to leave cause of an emergency
I assume the LFR group was successful in defeating the boss, because I got an item mailed to me and somehow I also got the achievement:
Cretaceous Collector:
Defeat Horridon in Throne of Thunder on Normal or Heroic difficulty without killing any Zandalari Dinomancers
Well, I left before any mobs even spawned, so I guess it makes sense that it would fufill the requirement of not killing any dinomancers.
But it was LFR; I still somehow got the achievement.
Perhaps with more testing, one can get other achievements which require u not to kill X things or stand in fire, etc, etc, if u leave an LFR group during a boss encounter and your group is successful in killing it.