Can you make yourself dream like nightmare with autosuggestion/mantras?
Are you asking if you can make a nightmare by using autosuggestion? I don’t see why not, it’s all about programing an intention in your mind. It’s just a matter of telling yourself you will confront whatever it is you want the nightmares about the next time you dream. Out of curiosity, why do you want to induce a nightmare?
Well, a friend and I have a “Who can get a nightmare first” competition s:
Dunno why we have it 
It’s possible, but I wouldn’t recommend it. I hate nightmares
Is it enough to say “I’m going to dream of something scary” or “I’m going to dream a nightmare”?
Here’s an idea:
Play Fatal Frame or Resident Evil (the first one on the GC) in the dark, alone, and on a rainy night (WITHOUT A GUIDE) for eight hours before you go to sleep.
That should give you a nightmare.
silent hill is better I think .
Nightmares are very usefull because you have more chances to realise ou are dreaming .