Very simple. Just get all headshots.

The furthest cultist, behind the bonfire, is pacing. Shoot him while he's moving.

Detonate all three cars by shooting their gas tanks. It seems kills are irrelevant. Vehicles in this game have a gas tank on both sides, so you can approach them from behind.

Straightforward yet deceptively finnicky, you have to shoot your bullet through the flames on the way to a kill. However, going through the higher flames often don't satisfy the challenge, so its best to shoot low.

No challenge. Enjoy your meal <3

Shoot both birds. They are both flying in a straight line back and forth, on either side of the... barn? Gazeebo? Barneebo?

Whatever. The building.

Shoot one of the cultists inside through the small opening of the front door. The back door does not work.

Kill two cultists with one bullet, by blowing up a gas tank. There is a truck parked on the opposite side of the camp from where you start, with one cultist who stands near it, and one who patrols past it. Wait for the patrolling enemy to come within the explosion's radius and fire.

Mark every enemy before firing. All are visible from your starting location.

Shoot through the bridge's broken roof. By shooting a bird when its near the hole, you can get a good shot at the back of the riot shield cultist.

Detonate the drifting truck to the left when its near the two cultists cheering them on, to where all three die to one shot. You can pull off the distant trickshot from your starting zone if you want to be cool, but the game gives you an easy out. The car between the celebrants is too far to kill anyone when you shoot its gas tanks, so you can loiter there after shooting it and be far closer to the drifting car when it approaches.

The truck in the parking lot will pull out onto the road after a minute or two. Wait for it to pull out and shoot the driver or the gas tank while they're leaving.

This one is awful. Good luck. You have to shoot a bullet from one side to the other, threading your bullet through two full train carts. The empty beds don't count, it has to be the actual carts. And even then, it sometimes doesn't trigger. If there's a more specific criteria, I'm unaware of it. You can use Trajectory Change to weave between carts, but I find that makes it less consistent to trigger and its even harder to time given the speed of the carts and the slowness of your bullet in that mode.

No challenge, only CPTSD night terrors.

Exactly what it says on the tin. Kill everyone with explosions, except the one inside. Start by powershotting the armored cultist inside, then hit all the gas tanks outside from there.

From what I can tell, this challenge triggers at 350 meters travelled TOTAL, rather than just between two points. Including my route just to be safe.

The armored cultist in the center of the building between you in the courtyard needs to be hit with a power shot as your first kill. As far as I can tell this level is unbeatable without the challenge.

On the first floor, three of the rooms are joint rooms, with the doors open between them. Send your bullet through all three interior connecting doorways, and the challenge will be satisfied.

No challenge.

You have to shoot through the psychic's bubble in such a way that your bullet hits the distant truck's gas tank without any redirection from you. Thank you to Intrepid Lancer and dpninja for the solution.

Send your bullet through the floating graves in the air and still complete the mission. Really simple.

The mission begins with a 45 second timer. When that finishes, four new enemies spawn in the parking lot, and the challenge is satisfied.

Kill both psychics immediately after each other with no other targets between.

Kill all enemies without hitting any non-targets (birds, gas tanks, etc). Relatively simple from there, though you may be spending a lot of time waiting for guys to get into place.

No challenge.