I agree with the idea of a spray or scent. My son went through a period of time when he had nightmares too. I told him that monsters, etc. HATED lavender and it keeps them away. (Just like bug repellent)... I put a bit of lavender oil on a small cloth every night before bedtime, we waved it around the room... he'd even come get me for a month or so if he discovered I'd forgotten to do it.
We also talked about a plan for the dream: how he could fight the bad guys/monsters. I let him know that although we don't hurt people in real life, in dreams, he could fight with whatever weapons he wanted and so we made up lots of little ways he could 'save the day' for himself. My son has a full-sized bed, so there were a few nights where one or the other of us slept with him. But between the lavender oil and the visualizations regarding how he would fight the bad guys/monsters in his dream, these both worked well. The visualizations were very empowering for him.