Full version of the question:
Given that the Elements of Harmony are locked to certain users, and the Crystal Heart at least semi-locked to a specific location, what other methods are there to defeat a Nightmare or Nightmare-like enemy?

For those who care for the background:
I primarily write changeling-based stories, and I settled some time back into a pattern with how I depict them. I've ended up with a story idea where that depiction is plot critical, but am unsure as to whether or not the resolution presenting itself to me is valid. The question of that resolution depends on a nightmare-related issue, hence why I wished to ask at your group.

I tend to think of the changelings as being divided into two distinct groups, 'normal' and 'darkling'. It is the darklings that I wished to bring into discussion.

As I understand it, the Nightmare itself was a separate entity, one which fed Luna's growing doubts and jealousy, eventually taking control of her - the union between the two being Nightmare Moon as she appeared in the show. Similarly, if I understand correctly (I have not read the comics myself), the same Nightmare entity struck at Rarity's doubts, using that to eventually take control of her, producing Nightmare Rarity.

Darklings, as I view them, are technically not Nightmares in that sense. To define what a darkling is, I'd offer this little thought experiment: Take that exact moment of utter despair when Luna originally allowed the Nightmare to consume her, and picture a changeling feeding off of and thereby being tainted by that spiritual darkness. Darklings, as I see them, are not possessed/controlled by a Nightmare entity, but rather can be said to have 'drowned in the darkness' - their consciences having been switched off, and their own darkest emotions and desires given full control, by the kind of destructive inner darkness the Nightmares create.