Use a different force besides the Archers, use the Dwarves or Redcliffe Knights you really don't need the Archers for dealing with the Arch Demon.
Secondly pull all of your party members up next to whatever Ballista you are using. If Leliana has a bow make her use that to target the Arch Demon.
To make her do this give a tactic like this.
Enemy: Elite Boss: Attack
Put that tactic in a fairly high spot in her Tactics and she'll prioritize the Arch Demon.
Wynne can be give can be given a similar tactic but set it below all of her healing tactics.
You might want to control Loghain and have him use the Ballistas to attack the Arch Demon, especially if your Warden has little trouble dealing with the horde of Darkspawn. --- "I love nailing Asari, so ageless and superior. Then you get them and they squeal like schoolgirls"- Gianna Parasini, Mass Effect 2.