I have a huge problem with this. In my nightmares, the gore is just incredible. When I have these kinds of nightmares, I usually wake up sweating, crying, and feel the need to throw up and my entire day is just ruined. I have tried the mental imagery and it doesn't seem to help very much. Some of these dreams start out very happy, like one started out with me and my husband holding hands and running through Disney World (where we had our honeymoon), and then we get separated and I witness all this violence and gore. It's crazy. The clonazepam I take does seem to make them less violent, but I still have them. I think it might be the suppressed rage, which, as my therapist has talked to me about, is the mask for the feelings of loneliness and sadness. When I identify my anger in that way and think about my emotions and what I'm feeling and why, my dreams tend to be a little better.
I remember when I tried this, I still had a nightmare, but it was just so ridiculous - I was playing pool volleyball with my husband (and this was extremely important because it was saving the world somehow) while a giant toy army guy (the green ones with connected feet) was stomping around in the pool trying to find us (but he had terrible eyesight, so we would simply move out of the way when he walked by). I woke up from that feeling anxious, but I mean, that's just kind of funny, I think. So, I recommend emotion-searching + drugs. Hahaha.