u/MrCreeper10K

So this is perhaps one of the most annoying challenges in the game, due to the target being constantly surrounded by enforcers, all of them with an "unique" AI.

I have, after having completed the challenge myself, found a way to "consistently" do this challenge (this is at least more precise than praying to the Hitman gods for good behavior from the NPCs, aka what most other guides recommend). Hopefully at least 1 person finds this post helpful. Let's go step by step:

1. Start at the garage, bring with you a lock pick, remote explosive and a tranquilizer gun.
2. Go out the gate in the corner next to the bog, and move towards the house, to the gate that leads directly into the shooting range. Unlock the gate and sneak onto the second floor of the barn.
3. Knock out the two guards there with your gun (or other non-lethal throwables, but you'll need to do it before they start talking because then they'll be looking at each other for a while).
4. Switch to the spec-ops disguise and place the remote explosive on the hay bale winch.
5. Now comes the finicky part. Unless I somehow messed with their behavior in my playtrough, the target and his entourage will be out in the open between the shooting range and the kitchen, and moving towards the house. Should be at about 2:40. Go down the ladder next to the winch, into the room below where the fire alarm is.
6. Wait for the guard patrolling that area to come and stand next to the ladder. When he leaves the room (passes the fire extinguisher on the wall) activate the fire alarm.
7. This will panic all the guards around you, including the enforcers following the target. They'll split off to guard specific locations, while the target (and two enforcers but as they're paniced, they won't start following you like they would normally) will be coming towards you. You need to grab the attention (don't get spotted) of the target while avoiding being spotted by the two enforcers. Run up to him, again, be careful not to let the suspicion bar fill up, and he should start following you.
8. Go back to the ladder room, while making sure the target's still following you, while making sure to not run into the enforcers. It's tricky, but if you do it right (and have a little bit of luck) you should be in the barn, bomb trigger in hand, and the target right where you want him.
9. Activate the bomb. Remember that the hay bale takes a second to fall so don't do it too late. While each attempt isn't long (if you save right before step 6 and the target is doing what he did for me) it requires more attention and quick action than most kills.
10. Escape. If you followed all these steps, you should be able to 100% "The Mercenary" in a single run.

Edit: Here's also a video, which shows a playtrough where the mission could be actually play out easier. I have no idea what caused this, every time I start this mission it's something new:

https://youtu.be/ZQLQ1pfHrIk