Your gunboat is slow. You're harassed by fighters the whole time, and you don't have enough payload to blow up the enemy capitol ship by yourself.
At your loadout, pick heavy rockets. These will maximize the amount of damage you can drop on the capitol ship.
At the very start of the mission transfer all your laser and ion cannon power to your shields, and transfer as much engine power to lasers and shields as you can. Keep transferring laser and ion power to your shields as your make your approach toward the capitol ship. You're going to fly really close to the capitol ship and you're going to need all the shields you can get.
Switch to heavy rockets and link your rocket launchers so they fire simultaneously. This will let you deliver your payload as fast as possible.
Hold you fire as you make your approach. Hold onto your rockets until you are literally right on top of the capitol ship. There is a lot of laser fire coming from the capitol ship and surrounding laser batteries. If you fire rockets too far away, they won't make it to their target because of all the laser fire.
As you get close, keep an eye on your shields. You might have to redirect shield energy to the front because you'll be taking a lot of fire on your way into making your bombing pass.
Let your rockets fly. Unload your whole payload as you fly right next to the ship. You might need to fly in a vertical S pattern timing your dips as your rockets fire. Flying straight makes it difficult to keep on target without ramming the capitol ship.
As you fly away, you'll likely have to shift your shields to rear as you'll take a lot of fire as your flying away from the fray. You won't have dropped enough damage to destroy the ship yourself, but you will have maximized the damage you can do making it easier for the other AI players to finish the job. You might even try issuing commands to the other AI ensuring that they also drop their payloads on the capitol ship to finish it off.
There might be another strategy involving Torpedoes that is viable if there is an imperial reloading skiff in the sector. If there is, you fly next to the skiff after firing all your torpedoes and issue a reload order to the skiff. Come to a complete stop and don't move your ship until it's done reloading your firing tubes.