If you're skilled enough, you don't need a party to play through the new Dragon Age — even on the hardest difficulty available, soloing Inquisition is totally possible.
If you're curious, he played as a rogue archer. At the start of the run, this meant a lot of kiting, keeping his distance, and and picking off enemies one-by-one. Later, as his abilities and damage progressed, he could afford to be bolder. In some of these videos, you see him doing over 2.5k damage per shot — it's incredible.
Playing still requires smarts, patience, and good positioning, especially given how easy it is to take huge amounts of damage on Nightmare, but there's still a difference between what the playthrough looks like early in the game, to what it looks like late in the game.
It's kind of funny to watch the final battle in the game played this way, too. (SPOILERS!!)
SPOILERS: since he can fade into the shadows, and since there are no other players there to aggro the enemies, there are many instances where Corypheus/enemies just stand there, waiting, not knowing what to do.
It doesn't seem like the game has been programmed to know how to handle being played solo, and that makes sense — most players won't play like this. There's a lot of joy to be found in playing with a party, most notably when it comes to banter. Obviously, players looking for a challenge can find party-less playthroughs to be equally fulfilling, though!
In terms of setup, this is what he was running on this solo nightmare run:
Death from above, leaping shot, long shot, rolling draw, strafing shots, full draw
Hidden blades, I was never here, throat cutter, knockout bomb, gaps in the armour
Stealth, evasion, evade, ambush
caltrops, looked like it hurt, cheap shot, throwing blades
For people like me, who suck at playing rogues, watching BOYvsVIDEOGAME go through the game like this is instructive. And if nothing else, watching him take on dragons by himself is really entertaining too. Not only did this player beat Dragon Age solo on the hardest difficulty, they also took out all the dragons in the game this way too. Here's one of them:
You can check out the entire run here, and all the solo dragon runs here. It's worth noting that rogue archer isn't the only one that can solo through things, in case you wanted to solo Dragon Age with a different class.