Is it possible? Or hopeless with just one CS?

You'll fully depend on the luck of the encounter draw. Just know that.

yeah without the third copy of many of the two of cards you are looking at some seriously punishing games and a hard slog to get any victories.

without a third copy of cards like feint, test of will, hasty stroke etc etc I would have little to no interest in playing Nightmare.

First cycle and the first half of the second cycle in nightmare are not so bad and you could easily still play those with one core but from mid way through Dwarrowdelf onwards Nightmare decks start to be some serious business and I personally would not enjoy playing with one core as you would get more or less smashed in the face by the encounter deck.

I personally bought a pack of cards that just gave x1 of all the x2 cards in the core set rather than a second whole core set and this has done the trick for me. I do still only have 1 copy of the 1 of cards but honestly the 2 of cards are far far far FAR more important and better than the one of cards. Many of the one of cards are either extremely situational, very high cost or are unique. The two of cards include cards like Steward, Feint, Test of Will, Hasty Stroke, Sneak Attack, Greeting, Forest Snare, Blade of Gondolin, Northern Tracker, Will of the West, Faramir, Ever Vigilant, thicket of spears and quick strike all of which can be utterly amazing in the right deck and only two of these cards are unique.....

The only x1 cards I would really worry about are UC and for certain decks/builds or solo play Henamarth Riversong. Celebrian's Stone I suppose is another one for certain Aragorn decks (next to useless in mine as Scorpigorn doesn't want more willpower and gaining the spirit sphere is useless to that deck) but honestly many of the x1 cards are very average and it is many of the x2 cards that you need a third copy of to make decks that have a bit more power and bite and have the potential to beat nightmare quests.

Long story short, I personally would not play nightmare without at least a third copy of all the x2 cards in the core set or it will be far too punishing.

You could use something like this deck: https://community.fantasyflightgames.com/topic/175783-one-deck-to-rule-them-all-nightmare-edition/

(Take a look at the end of the decklist for the single-core substitutions).

It can beat almost any Nightmare quest even in the single-core configuration. I think it's mainly due to the fact that Galadriel's Mirror can compensate for not having 3x or the key cards.

Sleeves and proxies!

I got a second core set awhile ago, and it helped a lot, but I don,t think I need a 3rd core set; I'd ratter spend the money to buy a few AP's.

A second core isn't that important for NM unless you're playing Spirit. I only have 1 core and I do just fine, but whatever floats your Dreamchaser.

You can try, but I wouldn't. 2 copies of the core is a must, in my humble opinion.

You can try, but I wouldn't. 2 copies of the core is a must, in my humble opinion.

Glad someone agrees with me!

We can't all make decks like Seastan!

I'm doing it ! Totally possible. Playing 2 handed, but one of the decks is primarily spirit, 1 core shared between both decks bam!

I don't have all, or even most of the NM quests, but many of them are totally beatable with 1 core. I haven't beat Encounter at Amon Din NM, but that's the only one that has completely stopped me. I've beat nightmares of Passage through Mirkwood, JAtA, Into the Pit, 7th Level, Escape from Darkness (another killer), and Assault on Osgiliath.

Don't listen to these nay sayers. With a little luck and perseverance you can prevail. Very rewarding when you do, I havent gotten all of the NM quests available because hey, it's hard enough on the pocketbook without buying all the NM quests, not to mention storage space.

Galadriel's Mirror is indeed the bomb. Galadrhim Weaver is a slam dunk card in this setup too.

Oh man. I thought he meant just 1 core set and no other player cards...

I honestly couldn't even imagine playing some of the newer regular quests without at least 6 cores, much less Nightmare. 4+ unexpected Courage on Beragond is a thing of beauty.