I've used the following solo deck to complete all the Nightmare and GenCon quests currently released.
Edit1: Based on feedback, I found swapping out 1x Arwen for 1x Unexpected Courage (which gets put on Galadriel) improves the deck slightly. Other than that, the deck I still use now to make videos is the same.
Edit2: Although I haven't done extensive testing, I think swapping out Daeron's Runes for Peace and Thought could also offer some improvement.
Total Cards: (50)
Hero: (3)
1x Glorfindel (Foundations of Stone)
1x Galadriel (Celebrimbor's Secret)
Ally: (10)
2x Arwen Undomiel (The Watcher in the Water)
3x Galadriel’s Handmaiden (Celebrimbor's Secret)
Attachment: (26)
3x Mirror of Galadriel (Celebrimbor's Secret)
3x Blood of Numenor (Heirs of Numenor)
3x Gondorian Fire (Assault on Osgiliath)
3x Gondorian Shield (The Steward's Fear)
2 x Asfaloth (Foundations of Stone)
Event: (14)
3x Elrond's Counsel (The Watcher in the Water)
2x Daeron's Runes (Foundations of Stone)
Sideboard:
1x Henamarth Riversong (Core Set) (For TWitW)
1x Will of the West (Core Set) (For For DDaCD and TLM)
3x Healing Herbs, Athelas, Waters of Nimrodel, Lore of Imladris (For AJtR)
If I only had access to one Core Set:
-1x Steward
-1x Dwarven Tomb
+1x Daeron's Runes
+1x Stand and Fight
Strategy:
For the majority of quests, the hardest part is surviving the initial rounds until you get set up. This is solved by putting a Gondorian Shield on Boromir, as it allows him to hold off enemies easily for several rounds. During this time you try to look for A Good Harvest and Steward of Gondor. Steward goes on Boromir and he starts to collect many resources. Eventually you attach Gondorian Fire and Blood of Numenor and he can now defend and kill pretty much anything. If you want to feel for yourself how incredibly powerful this combo is, try using it against Nightmare Conflict at the Carrock, and have Boromir defend all the trolls then take them all out the same turn they enter play. With multiple copies of Gondorian Fire he can easily take out The Watcher, The Balrog, Smaug, etc., all by himself.
Glorfindel and Galadriel help with the other aspect of the game - questing. With Nenya and a spirit ally in your opening hand, this deck can be questing for 10 on turn 1. But for quests that don't require this I tend to hold back and use Galadriel's card draw to set up Boromir. Her Mirror is responsible for bringing together Harvest and Steward in about 75% of my games. For location-heavy quests, use her Mirror to grab Asfaloth.
It doesn't look like there is much threat reduction in the deck given its dependence on Boromir. I initially had more but found it isn't needed. Between Galadriel, her Handmaidens, Elrond's Council (and Tomb), and Gandalf, I've only lost once or twice due to threat.
For a breakdown of the attempts, about 80% of the quests were beaten on the first attempt, 15% on the second, and 5% required more than 2 attempts. I could give a rundown of how each quest went, but most of the quests don't require any deviation from the above strategy and it would get repetitive (except for battle and siege quests but those are even easier because all you have to do is focus everything on setting up Boromir). But if there is interest, I can talk about the strategy for dealing with some of the more challenging quests.
Edited by Seastan