This was supposed to be fixed, but I have done the overthrow and killed the boss many many times now and it still won't drop the mote of light or count the kill towards the triumph. I've tried killing it both with a gun, and abilities. I've tried shooting the "corpse" during the death animation (someone had mentioned that could work), and I've tried standing right in his face while he was dying (another suggestion). Nothing is working. This is the last boss I have to kill, and it's getting a bit frustrating.