I found some useful tips to get this done;
Turn FOV to lowest, this will make your zoom and aim even better.
Use the goggles (ALT by default) and sometimes also aim further by pressing F in the goggles.
Use the vision thing as much as you can and need, this way you can see the enemies.
Take your time, do it slow.
Practice, Practice & Practice, this guide is only as good as you are, if you can't aim for sh*t, go through the mission a few times and practice each section so that you can get consistent at getting headshots.
VIDEO GUIDE: