Star Wars: Battlefront II/Preventative Measures

Destroy the enemy landing craft and repel the assault
These droids again... spawn as an Imperial Marine, and rush out to your hangar. Ignoring the droids (who are on infinite spawn anyway), whip out your rocket launcher and start pummeling that CIS Gunship. It might take you a couple tries, but the ship should fall quickly under rocket blasts. When that's done, simply mop up remaining droids.

Destroy 8 enemy fighters
Switch to Pilot, and hop onto a TIE Interceptor. These are fast and agile, making them the Empire's best dogfighters. Once outside, watch out for rocks—bumping into one kills you, and while they can be blasted to pieces, it's better to just avoid them. Remember to target slow ships like bombers and landing craft primarily, and you should take down 8 targets pretty quickly.

Destroy the enemy shields
Report back to your hangar and get in a TIE Bomber. You must overload the enemy shields to destroy them, and bombers are made for that. Getting close to the CIS flagship is the hardest part—once you're there, simply do some bombing runs over the top side of the ship. After three or four passes, the shields will fall.

Steal an enemy bomber and return it to our hangar
Don't worry if you don't know what a CIS bomber looks like; it's denoted with the ol' arrow. Land your bomber in their hangar, switch to the enemy bomber, and just make it back to your own hangar. Simple enough, eh?