This will work across all modes, I did this on nightmare. My party make up was Hero (Warrior, no specialization, was going for Champ on first playthrough), Leliana, Zevran, and Wynne. I did this with zero fire protection on any of my party members, so it would be much easier with some fire resist gear such as drake armor and the necklaces. Make sure you come with plenty of pots as this fight will take a while even when we're minimizing damage.
1. Spread your entire party out before summoning the dragon as the dragon will land on the hero and do take off a good chunk of life off everyone in range and knock everyone down.
2. Make everyone HOLD POSITION
3. Equip everyone with a ranged weapon.
4. Summon the dragon and chug a warming balm on the tank. Rest of your party members really won't need the balms. (I only had lesser, and it worked fine)
5. Immediately taunt with the tank. If you get hit, don't move back into melee range with the dragon. If you don't get knocked back, just step right outside of the dragon's melee range.
6. Have the tank face the dragon away from the party. You don't want to get the entire party fireballed or breathed on because that's pretty much a wipe.
7. Everyone should use their ranged attack on the dragon. Have the mage only cast healing spells.
8. The dragon will stay right outside your melee range and launch fireballs and breathe on the tank. Just keep taunting and shooting him while keeping the tank up with heals/pots. No one else should take any damage, and the warming balm will mitigate a large portion of the dragon's fire damage. Collect loot after he dies.
*The dragon may ocassionally leap into another position, just reposition your tank and repeat the strategy. It will take some time, but it's pretty much fool proof and you don't have to worry about the dragon's insane melee damage.
I was like level 8 or9 whenI killed the dragon I figured the ruins would keep him from flying up and wiping out my range dps. Well I was right the dragon got stuck in the ruins facing the wall. Still took forever to kill. Next play through I'll be staying away from the ruins.