You can get this in the first red zone already (so no need to camp in the other zones, because the enemies will get stronger and stronger in each area)
What i did was:
I would stay near a supply point in the pashall trainyard, so i wouldn't get detected by the kpa patrols and the airship aswell. So now comes the tough part, you will have to wait about 10-15 minutes each time.(watch a video/check facebook/do homework in the mean time i dont know, but dont forget to put the sound on for the notification!!!!!) I wouldn't recommend doing this on deathwish difficulty because you dont want to wait another 10-15 minutes because you died...
Also if the duster launcher flashpoint mission pops up, it will most likely glitch out and after a few seconds disappear (this has happened to me a couple of times, and there's nothing you can do about it) i just fast traveled back to the nearest safehouse and went back to my spot, and for the love of god.
DONT CAPTURE EVERY STRIKEPOINT! ESPECIALLY PASHALL TRAINYARDS IF YOU ARE FOLLOWING MY METHOD! I CAPTURED ABOUT 8 BUT STILL...
If you have any more questions feel free to pm me, or just send me a message on xbox itself. Peace!