Here's how I did it on nightmare difficulty.First off know that this fight can be a challenge,even at lower difficulties.For your party you should take a tank(aveline or warrior Hawke),a damage dealer(fenris,warrior/rogue hawke,possibly any of the other rogues),a healer(Anders or mage Hawke),and either a rogue or another mage as a damager(mage Hawke or Merril).Make sure everyone is at minimum Lv 18+.I used Fenris,aveline,mage Hawke and anders.Equip as much gear with fire resistance and physical resistance as possible.For weapons have everyone equip one that does cold damage.At least the mages,since they can always use Elemental weapon.Stock up on potions,especialy Mythal's favor and Life ward.
On nightmare the dragon is immune to fire,so don't bother with fireball or firestorm.Instead use winter's grasp as often as you can.Also the dragon has the following immunities on all difficulty levels,knockback,knockdown,immobile,stun,slow,paralyzed,enslavement,sleep,inevitable death,explosive death,frozen and brittle.It also has the highest health you may have seen thus far.
Start off by using a lifeward potion on everyone(don't forget to reapply when the effect wears off).Then activate sustained abilities,I used elemental weapon,rock armor,heroic aura.Have your tank use taunt on the dragon,and your DD attack.Have your mages cast winter's grasp then attack.Don't be afraid to use tempest,it does a decent amount of damage.Keep an eye on party members' health.Have Ander's heal when a party member's health drops between 50% and 25%.If your playing a mage you can toss a heal out in a pinch,if you know the spell.Use health potions if healing spells are still on the cooldown timer.
After you've done 15-25% of it's health in damage,the dragon summons dragonlings.And it will retreat onto a cliff.Dodge the firballs it spits,and kill off all the dragonlings.Afterwards the dragon can be attacked at range,until it comes down off the ledge.Continue the attack,and then it will summon another wave of dragonlings with a dragon.This usually occurs after its health drops to 50%.use the same method as before.You may end up doing 1-2 more times.
You can also summon you Mabari hound if you have acess to it.This fight will be long and it will be tough.You may even get a party wipe if you don't take care.But keep pressing the attack and it will fall.and you will get the cheevo.Also the dragon drops Fenris's last armor upgrade.