Tried looking the 3 vex wyverns with precision weapon damage in the perdition lost sector on Europa. There are two that spawn. Every time, I cleared the room of all other adds. I tried the belt and back. I would shoot them a couple times to open them up, then go invisible to snipe the belly. Didn't count. Went back, tried going invisible and going behind them to shoot the spot on the back. Didn't count. The triumph has to be bugged, or am I doing something wrong?