To get crowned you actually need to just deliver a single, killing blow to her head, which means she could be 90% dead already when you do it as long as its your first shot.

Crowning her (not for the achvmnt but just to get her out of the way) is so second nature to me now that I don't even skip a beat, sometimes forget to tell terrified teammates that I'm about to do it, sometimes happens so fast they don't even realize she was dead or even there to begin with, and unfortunately so second nature that sometimes I go to do it and don't realize I'm not reloaded, have a weapon thats not a shotgun and I get of course pwnt and look like a tard.