I have tried to do this achievement for several times now and for some reason (even though I am sure I only kill exactly the amount of mobs I need and no more) I don't get the achievement. Also made sure that I don't have any companion from Shado-Pan with me since they might fuck it up. Anyone else with the same issue?
It's definitely bugged more than just needing to be unmounted when you turn it in. I took me about 5 or 6 perfect attempts to get credit for it and when I did finally get credit for it I didn't do anything differently than the other attempts.
I did it without a Panda too, First time i did it it bugged and nothing, second time i made sure it was all down to me, i also wasn't mounted when i turned in and i got it
Also, don't share tags with other players. It took me a few attempts, but I finally got credit only when I was alone in the area and no one interfered with my kills.
My theory is if someone kills a mob with you, they become "linked" to you and they ruin your achievement when they go off and kill other mobs.