As with the Dark Passenger achievement the first mission seems to be the easiest mission to grind for this. It won’t take very long. The skulls (glowing blue neon look) can be picked up mid mission and you can get ten per run. To pick up skulls you need only fly over them. Be quick as they will disappear after a short time.
I believe to progress in the mission you need ten skulls (Fears) to continue on. So I don’t think you need to worry if you miss one.