Hokay, so after you complete the Nascent Dawn 5/5 questline, Ana gives you the Polaris Lance Catalyst, which astoundingly adds the Dragonfly perk to the already-explosive-happy Scout Rifle. Suhweet!

Problem is, Xenos and I couldn't get one of the quest elements to trigger, specifically the "50 Perfect Fifth kills" portion. The Perfect Fifth is the name of the extra explosive round that you get after landing four precision shots before reloading. We were killing dudes left and right with Perfect Fifth explosions, and we only managed to get a couple kills, but nothing seemed to be working.

I'm not sure if it's bugged, or if this is the intended behavior for the quest, but it appears that only enemies who burn out after being damaged by the Perfect Fifth explosion will count toward your quest. I've posted a small video below – the first Cabal is pretty simple to figure out, but in the second clip, ONLY the last Thrall counted toward my total. The others who died in the explosion did not count.

If you can, it helps to shoot the ground near several enemies, that way they take damage from the explosion and then hopefully the burn finishes them off. Also note that in this video, I have a Renown stack active, so my damage output is significantly reduced. Thrall tend to die completely in the explosion otherwise, so Acolytes and Vandals are probably your best bet. YMMV and all.