The Frostback is more difficult than its level belies, IMHO. She calls in her babies and flies around and perches on things for more variety than a lot of the other dragons.
How is your team set up in the tactics screen? I make sure that the warrior of your group follows him or herself rather than you. You might also have your rogue follow the warrior. Have your group set their mana/stamina reserve of 20%. If they are set to the default of 50% it really limits your team and how they cast their spells/abilities. I also change the health potion reserve to 1.
What might help is the fire resistance potion. If you get it fully upgraded, and have a team member drink it, it will protect the entire team via proximity. If you have any fire resistance belts those are helpful too. For some reason Dorian and Varric always like to stand in the line of fire, as it were, so I find I have to micromanage those two when dragon slaying.
+1. Fire tonic is very useful. You don't even need to fully upgrade it, just the basic level can increase your survivability long enough to do some firefighting (e.g., Dispel).
You should also gear up for fire resistance. You can buy Dragonling scales at the Crossroads market in the Hinterlands and make leg armor upgrades. Sometimes arms as well, but those are usually for attack.
Learn the attack pattern of the Frostback. It follows a set pattern and each position requires different tactics. For example, in a normal pattern, the dragon will fly up to each of the three upper perches. Two of them are inaccessible, so you either have to use ranged attacks to get her back down to ground level, or like I do, hide under the overhang so she can't firebreath you, so you can deal with her kids in relative safety. When they are all dead, she'll come down. The final perch is accessible, so you have to climb up there an do the final phase of the battle.
I say normal, because you can short-cut and eliminate some of the phases by knocking down her HP. The phases are triggered by HP level, like at 75%, 50%, 25%, etc. (just examples, I'm not sure of the exact numbers). If you take her from 90% to 30% in one attack, you skip two phases.