So one day ago I did it on a 5 players team this way: We all kept attacking her for over 30 seconds in the begining beginning and after it we started making the mirror mechanism.
Today I have done it only with an ED, he used the gfb on her 3 times and I did 2 times...But it was me the only one getting reward.
So I wonder that if you don't deal any real damage on her, the loot is random.
I will do it again tomorrow the way I did on the 5 players team to check