How do I kill the high dragon in Dragon Age 2?

As it's a fire dragon, it's extremely susceptible to cold: stock up on +cold damage items like Cold-Blooded or Mutiny, and try to get the Elemental Weapons spell.

But the fight is supposed to be a long one: it's got a huge amount of health. Health potions probably aren't going to cut it, and you should dedicate one of your party members to be the healer. Also make sure to stock up on fire resistance gear.

One cheap trick is to move to furthest point of the map when Highdragon is on the ground. When you are far enough your characters will go off from the battle. Then your characters will heal and you can save if you want to.