To unlock the Skeet Shooting trophy in RAGE 2, you will first need to play the Blackout mission, which is one of the three missions that become available once you access the open-world, and collect the shotgun after talking to Marshall.
Using the shotgun, you can use the secondary slug shot by simply aiming down the gun's sights before you shoot. When aiming down the sites, you will need to shoot an enemy that is in the air.
You can unlock the trophy towards the end of the Blackout mission when you enter the final room. Once you kill the final boss, some additional enemies will spawn. When they spawn, head to the bridge section and jump across to the opposite side. When one of the enemies attempts to jump the gap in the bridge, shoot them and the trophy will unlock.