Ambush! On the way to the final confrontation with the Rebel starshipyard, the Rage gets intercepted by group of Airam capital ships and fighters. To make matters worse, Rebel reinforcements arrive soon after. It appears that the Rebels have developed an Interdictor-type cruiser of their own!
Essentially this mission boils down to a clock management battle. In the TIE Avenger, focus on knocking out the R-41s of the Airam fleet, then intercepting Rebel B-Wings, preferably before they can get into torpedo range of the Rage. By the time the B-Wings have all been destroyed, the Calamari Cruisers will likely be in direct fire range of the Rage. Use your Mag Pulse warheads to prevent the cruisers from finishing the job that the B-Wings started.
Eventually the Assault Gunboats and the Rage will start having an effect on the Cruisers. Right about this time reinforcements, in the form of Super Star Destroyer Vengeance will arrive. Hold out until the last few Rebel ships withdraw or are destroyed, clean up the disabled dreadnoughts, and the mission is over. Congratulations! On the the penultimate test!