Mr C had awful nightmares when he came to me. This is what I did that worked for him, hope it's useful.
I held him for up to 40 min at a time in the night, very gently, not saying much and just being there. When he was a bit better he got a couple of very small strokes and he slowly went back to sleep.
I was superslow, and very quiet. He doesn't have them any more, but they took a few years to vanish completely. What happened in the end was that the touch of my hand calmed him down, and I got a grunty purr instead of a terrified cat. He hates foxes and if he hears one at night I am required to go to sleep with my hand on his back.