We're getting towards the end of the Rebel campaign here, and the difficulty curve, already high to begin with, has definitely taken a turn for the nigh impossible.
I'm not sure whether it's a bug or a design feature that killing capital ship components is basically impossible on Medium and Hard difficulties, but it definitely pushes a very difficult mission even more towards the impossible side. I highly recommend playing this mission on Easy, since I cannot believe that disallowing component destruction was an intentional feature.
In this mission, your only ship choice is the X-Wing. You have no warheads, a static asteroid base to defend, and a bunch of ships to inspect.
The inspection portion is simple enough, at least at first. While keeping an eye out for late arrivals, inspect all of the capital ships, and send your wingmen after the enemy T-Wings that jump in to engage the base. The T-Wings are armed with Proton torpedoes, and while they won't get in a lot of hits, each one will come back to bite you later.
Once you've inspected all the transport craft, help destroy the T-Wings. There's a total of twelve of them that jump in, after which the party really starts! A straggling Corellian Corvette will jump in, followed closely by an Imperial Star Destroyer, and groups of two Assault Gunboats. Send your wingmen after the gunboats, and inspect the Corvette quickly before the ISD can destroy it.
Try to position yourself to intercept some of the torpedoes that the Assault Gunboats launch at the R&D station. The gunboats will come in with six waves of two, and once they're finished, you'll still have to drive off the ISD, which will come into range of the R&D outpost to finish the job that the T-Wings and Assault Gunboats started.
This is where component destruction comes in really handy. If you're playing on Medium or Hard, your only choice is just going to be to try and hammer the ISD with everything you've got until it runs away. On Easy, you can take out the shield generators, then start targeting laser turrets until the ISD starts to go away.
Once the ImpStar is either destroyed or jumps away, you just have to wait for the remaining Rebel transports to jump out! Only a few missions left!