I have a spirit/lore deck that I have been using in solo play. It always beats Mirkwood, has a solid success rate against Journey (as long as you get Gandalf or Greeting in the first few turns), and gets thumped repeatedly during Escape. On a lark, I tried nightmare mode this evening. The result was a far easier game. Since I reliably end the scenario with threat in the teens or single digits, sustain no damage, and can recycle my discard pile into the deck, I was in a vastly stronger position at the end of the first scenario. Even though I discarded all of my allies and attachments, the low starting threat meant I had plenty of time to set up for Journey before the Hill troll engaged. I started the third scenario with 0 threat, and though my threat did hit 41, I eventually go the deck rolling, dropped my threat into the teens, fire bombed the Nazgul and fled into the night. However, I would have never been able to beat the scenario under "normal" difficulty.
Several people have mentioned the need to revamp the scoring system. I would go further, they need to revamp how they want to treat linked scenarios. Decks with healing, threat reduction, and card recursion can easily start scenarios in an improved position, which negates the whole point of higher difficulty. I would add that I was playing a deck that I frequently use in multiplayer, so I am sure that you could get even better results in a solo focused deck. For those who care, or want to experiment with the deck, I will post a list below.