lain_standalone: A solo nightmare playthrough requires a lot of time, meticulous planning and precise execution, especially in the early game. the amount of tedium is enough to test anyone's patience, composure and mental stamina.

When it comes to what feels like a real challenge to me, here's a set of rules I find reasonable:
-no mods except bugfixes
-no free (starting) DLC items, including tomes
-none of the following exploits: unlimited money, duplicate items, lvl up in Ostagar, item tier bug, trap-making in Lothering, level up using allied supply crates, mountain top experience exploit, unlimited enhancement crystals, fade essence/font exploit, free attribute points during a lvl up
-whether or not you want to limit yourself to increase the challenge and refuse to make money from herbalism.

Let's look at four build variations (to my knowledge) that one can follow. includes tactical commentary.
all of them (except hybrid ranger) are using dual daggers

0. the opening build. this is a very challenging part. whatever build you follow, all points from lvl 1-6 go into dexterity as it's crucial to increase your chances of dealing damage and avoiding damage from attacks, therefore saving as many health poultices, bombs for Gazarath (you can also lay some small claw traps in advance)/Ogre. to get to korcari wilds you must first finish dwarf noble origin story. which is ridiculously hard... you will have to defeat the mercenary captain to progress. this guy won't use any talents, but hits for ~20 damage (you have 110 heath), runs slightly faster than you and has ~300 health. you hit him for ~5-8 damage with your attacks and ~15 with dual-weapon sweep. that makes ~21 dirty fighting into dual weapon sweeps combinations to take him down without taking damage. to do that, you need to lure him to an area with the darkspawn bonfire and run around it, kiting him and waiting for your talents to cooldown. he's a lieutenant rank foe, so stun duration is reduced, that means you only have time to perform a dual-weapon sweep after dirty fighting and must immediately resort to kiting around the bonfire. the blightwolf fight is identical. korcari wilds is quite tedious but remember that you need to pull with a longbow as it has more range. fight the monsters one by one and kite hurlocks around objects (they are very strong and have a lot of health), use dirty fighting, do a couple of backstabs into dual-weapon sweep and save as many health poultices and bombs as you can. this is crucial. I don't recommend taking on Gazarath if it's your first solo nightmare attempt. the Ogre fight is about how good is your circling movement, how many bombs and health poultices have you got and how many rams in a row he uses. when he rams, run to the side to end up far away from him after the ramming. if he grabs you, you're dead. there are three ways to evade the grab - run around in circles so that the grab conditions are not met for the AI, use dirty fighting to ministun and cancel the grab and bait him into grabbing by dancing in and out of his melee range. the animation will start but you're too far away and the grab is cancelled. good luck.

1. dexterity build
this build has a powercurve that starts to make a noticeable difference by ~lvl8 ( so, in soldier's peak, honnleath and return to Ostagar), making the progress much less tedious (less kiting and dancing around objects, less health poultices and bombs used, less small claw traps used) as you'll have high defence and attack rating, therefore getting less hit by enemies and landing more hits when attacking them. you'll need 18 cunning by lvl 8 for combat stealth, rest of the points go to dexterity. downsides - you won't be able to open chests, you won't be able to persuade/intimidate until lategame (lvl 16+). you will get a maximum of 9 damage from tainted blade. if you picked bard, you'd only get 5 attack/3 damage/5% critical chance from song of courage. therefore, assassin's 2 attack, 2 defence, 2.5% crit chance, 3.4 backstab damage (though I don't recommend wasting a talent point on exploit weakness) and 20% damage increase is a stronger option.

2. cunning build
early game becomes more tedious, but you get more loot and can persuade/intimidate if you put a point in coercion early on and sacrifice herbalism (not recommended unless you're doing no money making from herbalism run). you need 24 dex for momentum, rest goes to cunning. you get ~25 damage from the tainted blade at lvl 14. you get 8 attack, 5 damage and 8% critical chance at lvl 14. the dexterity build is stronger early-midgame, cunning build is stronger lategame.

3. 1:1 dexterity: cunning
gets the good from both builds but sacrifices mark of death. you'll have ~10 less offence/defence rating midgame, ~12 damage from tainted blade by midgame and 6.5 attack, 4 damage and 6.5% critical chance by lvl 14.

4. hybrid ranger build (this is a build i never tried. there's a playthrough on youtube, but he's using the free DLC items thus making the build questionable.)
it opens up with a dwarf commoner origin, uses a summon, a longsword (asturian's might) and a dagger early on and starfang with veshialle later on. 24 str, 36 dex, rest in cunning. I will not post the skill/talent build here as I am not interested in this build. do your own research.

Specialization
7 duelist/14 bard (cunning build)
7 duelist/14 assassin (dex build)
7 bard/14 duelist (1:1 build)
7 ranger/14 duelist (hybrid ranger build)

Skills (all builds)
1. trap-making
2. poison-making
4. herbalism or combat training
6. combat training or herbalism
8. combat training
Old Tegin tome. herbalism
10. herbalism
12. poison making
rest of the points stealing/coercion/herbalism/trap-making

Talents
Cunning build, 1:1 build
1. below the belt, dual-weapon sweep
2. stealth
3. dual-weapon training
4. stealthy item use
5. flurry
6. momentum(if you choose to delay herbalism)/deadly strike
7. deadly strike (or nothing if you plan to pick momentum at lvl.8)
8,joining ritual. lethality, combat stealth, momentum (if you didn't pick it earlier)
9. dual-weapon finesse
10,tome,tome,tome. dual striking, riposte, combat movement, coup de grace
11.
12. master stealth
13.
14. song of valor, distraction, song of courage
15. duelist
16. upset balance
17. keen defence

Dexterity build
1. below the belt, dual-weapon sweep
2. stealth
3. dual-weapon training
4. stealthy item use
5. flurry
6. momentum(if you choose to delay herbalism)/deadly strike
7. deadly strike (or nothing if you plan to pick momentum at lvl.8)
8,joining ritual. lethality, combat stealth, momentum (if you didn't pick it earlier)
9. dual-weapon finesse
10,tome,tome,tome. dual striking, riposte, combat movement, coup de grace
11. mark of death
12. master stealth
13. duelist
14. upset balance
15. keen defence

Quest order (after Lothering)Soldier's peak->Honnleath->Return to Ostagar->Circle tower->Redcliffe->Urn of sacred ashes->Denerim sidequesting->Orzammar, Cadash thaig->Brecilian forest

Items and gear
you need to buy all the health poultices, bombs and metal shards, elfroots and deep mushrooms early-midgame. all the fire crystals, frost rocks, lifestones, spirit shards, frozen lightning and corruptor agent you feel like you can afford.
if you decide not to make money from making greater lyrium potions, you will have to make some choices. you will most certainly need to buy all the talent and skill tomes available. when it comes to your first big investment I reccomend getting the Spellward and dweomer runes. it's possible to get The Rose's Thorn lategame. it's also possible to get The Felon's Coat after Landsmeet.