Light 'Em Up is one of the hidden treasures in Far Cry: New Dawn. Just like with the previous missions of this type, you can learn about this one in Prosperity, in one of the captured outposts, or while you are exploring the world. The exact location of the mission is presented in the picture below:
Reach the place and eliminate all bandits. There are three of them. One bandit is standing a few meters away from the other to - you shouldn't have problems with eliminating him with a stealth kill. The other two bandits are in the back of the building.
Enter the building to locate a Wolverine's lair. Go through the building. The animal is in a locked part of the building. Leave and set the building on fire - use a Molotov cocktail.
The locked part of the building is now open. Go there and find the Wolverine. If it didn't die due to the flames, shoot it to get Highwayman Shed Key. Use the item to open the barn near the building. Inside, you will find resources, ammo and perk points.