Was easy for me. Did it first try. First change your targeting mode to expert...and leave it. Auto aim sucks. I start by targeting there hat...click on dead eye. Shoot off there head and rotate the stick in counter clock wise pattern while aiming at there gun. Pull right trigger and BAM! Easy. Try shooting the hat off of a person while riding full speed. You only have about 1 second to do it before your horse slows down. o.o Pretty challenging...but did that to.
I actually do it by accident all the time. Using casual targeting I usually auto aim and tilt up to aim at the head but go to high and shoot the hat off and then the next time I shoot I tend to aim down a bit and I find myself disarming them.