Guild Wars 2

In total i should have this achievement 3 times. But it seems bugget. Two times i throw 8 orbs on liadry and then got downed and kill her (i got reverd for killing her), budt didnt get achiev. So i thing mayby i counted wrong so i tryed again. On my last attemt i hit her with 9 orbs and when timer reach zero i kill her. Again i was on halo with reward for killing her but with no achiev.
I know this is hard to prove without video but i atleast want ansver on question. “What should i do to get this achiev?”

try different arena i read somewhere that the slyvari arena is bugged.

Thanks it realy was becoase sylvari arena is bugged