I killed the Frostback at level 9. It was long and hard but there is a trick to doing this.
First you need to run around in the first valley and use your rouge with the Hide ability. This will let you avoid the dragonlings and the dragon but he will still aggro on your teammates. You are not trying to even hit him at this point.
Then keep going until you find the pass at the back that leads to the new area.
After he gets tired of throwing fireballs at your teammates and you get to the back he should come and land in the back area.
This is when you switch to your mage. Make sure the mage has fire resistant gear and an ice weapon with ice spells..
Have everyone in your party run at the dragon and attack it. They will all die but this is just to keep him busy while your mage runs around him.
If you go around the dragon and towards the docks where the boats are there is a little crag of stone on the left that comes out and hooks around at the base of the big upper area in the middle. This crag is the key to beating the dragon.
If you position your mage correctly you should be mostly behind the stone crag but still able to fire past it. The dragons fireballs are very big and will hit the stones. Don't stand to close to them. You will be shielded and your frost shots will hit the dragon slowly tearing him down.
Also if the dragon is too far away for the targeting to appear, manually angle your shots carefully at the dragon. If they hit they will still do damage even though the small white plus is not visible indicating the auto targeting is helping you hit.
Occasionally he will go very close to the middle section and be in an angle where you can't use the crag for cover and still hit him. In this case just run out hit him with your heaviest yet fast spell and quickly duck back before the fireball.
Also, when you are behind the crag if your angle is right you won't be affected by the suction ability but you may want to try and anticipate it and run closer to the wall when he looks like he is about to use it.
Great overhead picture thanks to a glitch someone had.. The spot I am describing is in the red circled area.
