UPDATE: The last DLC 'Trespasser' has the achievementwhich requires you to kill the dragon described in this solution when your entire party is at level 16. So, if you intend to 100% this game, wait until then if you want to save yourself an extra playthrough!The first and easiest dragon to kill is the
Ferelden Frostback, who can be found in the Hinterlands, just north from Dusklight Camp. He's a level 13, so to beat him comfortably, it's advisable to wait until you're about the same level.
I personally defeated the beast when I was level 13, playing on hard. My party consisted of two warriors (Herald and Cassandra), Varric and Vivienne.
Fereldan Frostback is vulnerable to Cold, so equip your mage with an ice staff and teach him at least
Winter's Grasp. You can give yourself an extra edge by enchanting your other party member's weapons with frost runes for some extra cold damage. Should you have fire resistance increasing gear, equip it. And upgrading the amount of potions you can carry, as well as their potency, will help a lot.
For the fight to begin, you must first run through the gulch crawling with Dragonlings, while FF bombards you with fire balls. Try to not get hit by jumping and using the environment for cover. If you have to use a potion, it might be better to return to camp for restocking, and start a new run.
During the fight, FF main attack will be his fire breath, but he won't be able to use it if your characters are close. His wing attack is nasty, as it will create a vortex that deal a ton of damage to all
outside the vortex - another reason to stay close to mid-range. He also kicks those hacking at his legs, but that can't really be avoided and does comparably little damage.
You can attack each leg, but I advise to focus on either front leg, and then on the back leg of the same side, as this seems to shake his balance, making him more vulnerable. Frequently pause to make sure your heroes attack the proper leg, and see that your mage casts
Barrier as much as possible. Once you damage him enough, FF will take off (twice) and start blasting you from above. Again, run and take cover until he returns, the continue.
The fight will enter it's second phase once he calls his minions for help. Dispatch them quickly with two characters while keeping him busy with the other two, then resume your full attacks. He'll fly off once more and try to hide on a ledge, where you have to pursuit him. Keep at it, and eventually, after what will seem like an eternity, he'll perish. Hooray for loot and a morbid throne!