Injuries in Dragon Age 2

According to the Dragon Age Wiki, it sounds like the character that dies is based on Hawke's class. This is probably to aid in giving the player a balanced party without too many redundant classes.

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.