First and Foremost, learn his pattern, this is the most important thing. Demon commander is very simple. He warps underground, and then whoever has aggro is marked, by a black and green smoke. Not to be confused with the greenish mist that is a wraith's Weaken. A second or two later he will do the uppercut. After a few moments, sometimes almost immediately if you manage to dodge and then hit him, he will do his scream, the scream is three blasts, can do damage. Initially it slows you, and if you get hit by last blast, it panics you, for a while. He then tunnels again and pops up at a distance to shoot for spirit bolts at whomever has aggro. After this he tunnels for his uppercut, and its rinse and repeat.
Strategy
First and foremost, you want to stay in the beginning room, but do not bunch up, have warrior types near door but again away from each other, and not actually blocking doorway. Then you want ranged chars back a bit firing out the door, their main idea is to get wraiths to bunch together so they can wipe them out at a distance, chainlighting, walking bomb and other such multi target abilities are great here. This is you base set up. Try and be aware of who has Aggro, and always, always watch for the smoke. A good strategy is watch for him to go under after he does that, just turn and run in a dirrection till he pops up.
Mages: Barrier and Fadestep or fade cloak are the way to go, I personally have fallen in love with Fadestep, costs no mana and you move fast, as long as you pay attention, and react fast, DC will never hit you. For Arcane Warriors, if you draw aggro, when he shoots at you, knock the blasts back at him for decent damage, and if he uppercuts on another member of the team, this is the time to fade step through him, turn and cut him to pieces, you might get hit with the scream but thats not too bad.
Shield Warriors: Best strat is to either run, or use shield wall or walking fortress, its a bit tricky, but if you play with it you can come out in the positive, payback strike is good for getting out of the panic.
Two handers: Reaver, best strat is to run to avoid the uppercut, if you can, immediately turn, hit rampage, and go to town with Dragon Rage, your strat should be to hunt him down and beast him, kill him as fast as possible. Katari, Running bull is your bread and butter here, if you're targeted, use bull to run when you see the smoke. If you're not targeted, then use bull and run through him, you won't do much damage, but you'll hit him and get guard, win win.
Dual wield: Stealth, Stealth, Stealth. If you do things right, DC will never target you. When I play my assassin, I actually stealth out of the room, my goal is to run around and clear off as many wraiths as I can, as well as hitting some of the other mobs, the idea is to keep them off your teamates backs, if you do it right, you'll never be unstealthed for more than a second.
Bows: Distance and leaping shot, save your leaping shot and manage your stamina, that way if you get targeted, the moment you see black smoke hit leaping shot, if done right you will avoid his strike and get in a good hit.
These are just the strategies I use, and I tend to do pretty decent with them, mind you I'm only up to playing threatening at this point.