Polarity, destroying weak points before killing.
This one was the worst for me, there's probably some decent way of doing it, if anyone knows.
I tried using the chem launcher corrosive and the firefly demolisher. After many variations, I ended up using the riot foam chem launcher, a grenade, the firefly demolisher, a tier six sill build with the acosta go bag. I would start the washington hotel mission on normal, riot foam the three enemies that come out, grenade them to lower their health and get overcharge, then firefly them. This managed to kill an enemy maybe once every other try. If I didn't use the grenade, none would die. I also couldn't do any follow-up firefly shots. I could only use it once. On any follow-ups, I could riot foam an enemy, stand right in their face and launch the firefly, and it would go off in some other direction. It's almost like it knows they have no weakpoints anymore so it ignores them. But if I kill them with a weapon at that point, it wouldn't count towards the task.
So there's probably some easier method and I'll try to remember it for next time. Anyone have one?