I've only done diamond ARs, but I found it easiest to focus on getting through all the headshot challenges first rather than gold 1 gun at a time. I got in a rhythm of getting headshots. Then I did all no perk/attach. Usually I was in roughly the same spot after no perk/attach. I'd need a couple double kills, 4-5 blood thirsties, and 8-10 long shots. I focused on longshots next and I learned spots on all the maps I could get them. Then I just played normal for the rest.