Attack the archdemon until it moves from the center to one of the other platforms. The beast will summon darkspawn so use whatever armies you have that has the highest soldier count (perferably all 50). Move to the closest ballista that the archdemon is near and just keep shooting. When it's health drops again (to 50%), it will move to another area. You can't use melee attacks so just get to the closest ballista to the archdemon and keeping hiting it, even if it gets REALLY boring and you think the hits aren't working, keep at it. When it's health drops a final time (to 25%) it will move to another area. You can either move to another ballista or just take whatever NPC that hasn't died and attack the dragon until it finally dies. Still having problems? If you're a Tank, play the fight as a Rogue or Mage. Your low armor will draw less fodder from the darkspawn. If you want to change your NPC's statics, depending on how well you developed those character's abilities, give them (or take away) health potions so you can use them on yourself. Doesn't matter if Alistair or Loghan die (or that you die), just as long as one party member kills the archdemon, a cutscene will occur and you will win the game.
