There's the obvious of course....lower the difficulty.
Now the more enlightened response:
You can make the fight a lot easier by focusing on the darkspawn instead of the ArchDemon. What I mean is, when the battle first starts it's just your team and ol' meeny. After you do some damage, he'll fly off and call a bunch of guys to join the fight. At that time, run your team off to one side (I think it's actually top or bottom) of the battle area, preferably the opposite direction of where the AD went. You can then concentrate on taking out all of the darkspawn on that side without the AD interfering. If you do that and then do the same on the other side it'll just be you and the AD again, no support. Otherwise, the darkspwn just pick you off while you are trying to kill the AD.
Also, you get a helm during the first part at Denerim gates I believe called the Corrupter or something. It gives one character 75% resist to spirit, making them nearly immune to the AD. Make sure to put it on whoever is going head to head with him.
And regardless, you are definitely going to need SOME healing potions. I hope you have a few anyway.