Thanks. I finally figured that out. The first two times I tried this I had 19/20 when the boss target went down and the mission completed popped up on the screen, but in both cases there were still enemies on the area and got the 20th headshot while securing the area. Because that happened while in the mission area, but after the boss died, it reset back to 0 and did not complete the challenge. Regardless of the number of the enemies that are alive in the mission, if the boss goes down then the mission/challenge is over. Lesson learned: Just get the challenge out of the way early in missions.
Hello LeftyLebrandt,
Thanks for bringing this to our attention!
I'm glad that you were able to complete this challenge. If you still have any other issues with any other challenges, can you please provide us with a short video of this occurring? Many thanks!
Additionally, if you have any other questions or concerns, please let us know.