THE FINALS

You can get this in any mode but personally I felt like the new "Power Shift" mode makes for a good place to hunt for this if you're trying.
This is because everyone is always grouping up together into the same place so you can have up to 5 targets available at once while you can take advantage of this and play carefully in the back or to the side.

Personally I got this with the Medium's "R .357" which is the pistol. It does pretty good damage and only has 6 bullets so you can count or even just waste the first 5 and try to get a headshot while hiding off to the side and everyone else is fighting.

Though, especially for this mode I'd also recommend trying the Light's "SR-84." This is a powerful sniper weapon with only 6 bullets again and it's very easy for lights to tuck away on a roof somewhere and snipe down at people on the platforms, making this a great option as well.