So, as part of the C’Thun event in Mercenaries, we’ve got the task ‘Famliy Gathering’, which reads:
Complete Onyxia Bounty with at least 3 Dragons in Onyxia’s Lair.
I’ve done it on heroic with a party of six dragons (lost one along the way and another during the boss fight, but that shouldn’t be a problem), however, that didn’t count.
[Updated] Workaround: do it on normal instead.
Are you doing the Undead Onyxia fight? Maybe you’re accidently doing the wrong Onyxia!
Many people have successfully completed the event based on OldGuardian’s guide.
-Valas
Are you trying to suggest I’m that illiterate, dumb or attention-deficient, or are you trying to promote some guru wannabe ‘OldGuardian’ with their guide(s)? 
By the way, the event is still ongoing… or it should be, as far as I’m aware, you probably mean completing the task in question, don’t you?
Oh, and one last small thing: it did count completing the task on normal. In my experience with the previous events and even the first task of this event, you could generally do either a normal or a heroic bounty, unless specified, if you wanted to, however, for this one, apparently, the heroic version is not properly accounted for, which seems like an inconsistency and a bug. So, as of this writing, my suggestion (and the workaround to complete the task) would be: do not go to Onyxia’s bounty on Heroic due to the issue described above, do it on normal difficulty instead.
I suggest none of the above!
OG’s community is fairly quick to comment and report situations that arise, which would have supported your situation if there was in fact a problem. None of them were reporting anything as his guides always suggest to take the easy route and complete it on Normal.
When I said “event”, I was referring to the task in question. Error on my part.
My final suggestion would have been to try it on Normal, but that was an afterthought for me and I didn’t bother to come back and include it. Glad you were able to resolve the issue!
-Valas
Nevermind a bit of irony above… Oh, and looks like your post is being edited really fast, even as I’m writing this!
I must insisit that there is, in fact, a problem with the task in question, based on the reasoning above, and the purpose of this topic was to report a problem (by the way, the task hasn’t been out for that long, at least on the European server where I’ve done it — and Americas are event behind in terms of time zones, so it’s probably only Asia that has had plenty of time to play-test it, and thus I thought it worthwile to report), not to seek guidance, but thank you anyway.
No worries and best to you!
Still waiting for the task to release in America myself. Appreciate your insights prior to attempting myself.
-Valas
Oh, by the way, I’ve taken a look, and the community you’ve mentioned is also reporting the bug, while the ‘guru’ is asking just the same thing as you. 
That guru and his community need to step their game up!! Managed to complete it myself with your insights 
-Valas