Salvation is here, people. Here comes the guide.
First of all, I like the fact that the challenge works like "if they didn't say it, that means it's fine".
So first, instead of the Chapel, I used this little camp with supply helicopter side mission. We have 3-4 Heavies in the area and an electrical generator for your TF Challenge. You can deploy at Espiritu Santo Delta. It's just around the corner.
I parked behind an enormous tree where I can get a vision on the watchtower to kill the Sniper before he made any trouble.
Follow this method closely. I activated the drone and flew it
facing downwards like in the picture. I measured the drone to be hovering less than 20m above people's head using my teammate first.
Once I got it, I hovered to the camp carefully while still facing downwards. There was one Heavy who stayed at the sandbags barrier. The rest were walking around. If you manage to get your drone to spot them within 20m above their heads, then soldier, you have fulfilled your requirement.
If you screwed it up, run, and don't die. If the game decided to mark the Heavy while you were still more than 20m away, then just get what you can get. Either way, survive and redeploy at the Espiritu Santo Delta to 'flush' the scene without resetting the count. As long as you don't die, you're all good.
This is overview of the camp just to make it easier for you. Not all enemies been marked.
