Are you by chance blasting the civilian yachts hovering over the city? DON'T, your suppose to escort the Y-Wings who use their ion cannons to disable them, it is required that the yachts survive. you should concentrate on destroying all ground targets and ties. Here is one walkthrough written for this mission:
Level 5: Liberation of Gerard V
Escort Y-Wings to their targets
Disable civilian yachts
Destroy Tower Turbolasers
As you fly toward the first city, you'll see two Turbolasers on the ground to your right. Slow down and take them both out with Blaster fire in one pass (if you miss one, don't make another pass to finish it off just yet). Next, close your S-foils and continue in the direction you were going before turning to shoot at the Turbolasers. You will come to a Missile Turret atop the hillside surrounding the city. Destroy the Missile Turret with your Blasters and continue flying to the right; along the hilltop. You will encounter two more Missile Turrets, one right after the other. Eliminate them both in one pass and then proceed to center of the city. Now take out the two Turbolasers mounted on top of the buildings.
You'll need to make quick work of these Missile Turrets before they knock out your Y-Wings. Once all those deadly turrets are gone, the Y-Wings will have a much easier time disabling the blue civilian yachts. Now it's just a matter of time before the next threat appears. In the meantime, focus on destroying the other Turbolasers and AT-PTs throughout the city. Soon you'll receive a communiqué from Wedge indicating that another section of the city is being attacked by the 128th elite TIE Interceptor group. Follow your fellow wingmen over to the next section, taking out the two AT-PTs along the way. As soon as the next section is in view, you will see several more Turbolasers and Missile Turrets. Ignore the Turbolasers for now and focus on eliminating the Missile Turrets as you make your way to the opposite end of the city. There you will find a Tower Turbolaser. Take this large turret out as quickly as possible with several Proton Torpedoes and lots of Blaster Cannon fire. After it blows up, go back and provide more cover for your Y-Wings by taking out the remaining Turbolasers and Missile Turrets. Before you know it, all the yachts will be disabled and your mission will be complete.
Going for the Gold
To earn the gold medal, you'll need to destroy a significant number of enemies. After you have destroyed the Tower Turbolaser and all surrounding Missile and Turbolaser turrets in the second section of the city, quickly shoot down several TIE Interceptors to bring your kill count up.
You'll need to finish the level in a scant five minutes and 15 seconds. Don't waste your time with multiple attack runs. Instead, plan your route in advance. With your finger on the brakes, try to take as many Missile Turrets and Turbolasers as you can in a single pass. Use your Proton Torpedoes if you need to.
Requirements for Gold
Ties are flies, swat'em!