Every time I play this, my squadron and I manage to blow up every enemy fighter and bomber and allow every employee ship to escape but as soon as Dunari himself makes a run for it, the Star Destroyer pounds his ship into dust! What am I to do?

Frankly, I don't remember this mission, but why not just attack the Star Destroyer? Take out its warhead launcher so it doesn't shoot missiles at you, and just pound it, with help from the rest of your flight group. You'll either take it down, force it to withdraw, or at least draw its fire if it decides to fight back.

When you enter the Casino Area you are ordered to attack the fighter coming from the left. before you are completely there 2 capital ships arrive, the Star Destroyer and a cruiser, (forgot which one) You hav eto go to the ISD immideatly and try to take it out as fast as possible. You have to make several attack runs for that, so every time you move away from the ISD go into the direction of the casino while taking out fighers. Insave distance of the ISD turn around and make the next attack run.

Remember the Shield Generators on top of the ISD! When you make the attack run, try starting on the front so you have to fly a while over the entire hull of the ISD. If you fly in a straight Line to the Shield generators, the ISD Guns are unable to hit you, if you fly fast(sometimes lowering shields helps) Make blank shots on the shield generator, so no targeting the ISD.
3 torpedos, or 5 missiles will take out 1 Shield generator.