At this stage of game, you should have at least 9 greater spirit balms (you can collect spirit shards and have Morrigan make them, or you should have been able to buy that many by that stage. In my set-up, it's actually not necessary, as PC warden and Zevran both have over 50% spell protection (if PC warden is a warrior, then 75% protection), but lesser spirit balms seem to last shorter. You give one to each member who is not wearing the helmet at each stage of the fight, and I haven't actually missed Wynne when I took Loghain instead. (That said, Morrigan's second spec was a spirit healer and she had heal and group heal)
This is probably the only fight I change the group tactics completely. The entire group either attacks Archdemon (Mana clash works well and paralyze also works so does petrify) or protects the one who is attacking him. Your warrior and rogue both have para runes in the main hand (in the case of a rogue, three para runes), to keep it incapacitated as often as possible. Basically one party member (usually Zevran because I give him at least critical shot just for the second stage) is told to ignore elite or less as a secondary target and just attack the boss (duel weapon sweep, flurry, or just plain backstabbing) and the other members protect him, sending those attacking the party member to sleep or waking nightmare on them, mass paralysis, war cry, just plain melee attack, etc. (In the case of a warrior, he is also told to intercept when the mages are attacked by melee). I don't need to call for allies at all for the first stage. The arch demon doesn't last long. The second stage, I will call dwarves to deal with darkspawns that come to aid the archdemon so that the party can solely concentrate on the archdemon and the one who is shooting arrows at him. If they are distracted, just move them manually near the party member. The allies are there to deal with the other darkspawns. (Apart from positioning them near the archdemon and give them greater spirit balms, I don't interfere at all, which is a good thing, because it used to crash all the time when I tried to pause the game every so often.)
The third stage is the fun part, everyone can have a go at the arch demon again. And backstab is a wonderful thing against any boss. Rogues can last and do very well at this stage, even on nightmare setting, provided that you built them up right and gave them proper equipment to do the job.