Choose a different plane. The A-10 is next to useless for this mission.
Try the TND or the Raptor.
You have to be cycle thru the targets while you are out of range.
Destroy the things that will shoot at you.
In order of importance: SAMs, AA guns, Launchers.
The advantage of the TND is that with the spread bomb, If you line the Sub up from bow to stern so that you fly over its length, you should be able to destroy all Hrimfaxi's offensive weaponry in a few passes.
Also, you don't have to "head for the skies" until the AWACS plane starts the countdown. Then go vertical and full throttle.
Is this your first time thru the game? It will only get marginally easier with repeated replays.