I tried to complete this Special Assignment yesterday on one toon. I picked up the sniper rifle, aimed at the Darkfuse, and pulled the trigger. I could hear the gun shot but nothing happened to mob. I tried on multiple mobs. I tried moving the target reticle around to aim with different points of the reticle and still nothing.
I have left sniper mode and then re-entered and still no fix. I went far away from the WQ site and returned to see if it would reset and still the same issue. I logged out and back in and still cannot complete the Special Assignment.
I was able to successfully complete this on a different toon the day before.