Headshot a total of 100 enemies using the Wingstick.

You will acquire Wingsticks at the very start of the game and can begin using them immediately. They at first will just be thrown with 'RB' and will track and hit a target if they are in an open enough space. Typically the Wingstick seemed to go through the bodies of enemies and hitting headshots was purely luck based. You can upgrade the Wingsticks via the 'Projects' tab in the menu in the 'Capture & Control' section allowing upgrades such as 'Wingstick Lock-On' and 'Fast Wingstick Lock-On', these will allow you to hold 'RB' with the Wingstick equipped and you will get a red flight arc that shows you where the Wingstick is going to go, if you aim above and down onto enemies, the lock-on tends to lock onto their heads, though it's not always the case, especially if they're moving.

I personally found this easiest to grind in Mutant Bash TV as the enemies here are weak, but you also get an on-screen prompt saying 'Headshot' when you get a headshot, so you can use this to try and gauge when you are getting successful headshots.