I'm currently playing Wing Commander 1, and I got to the mission where you're supposed to defend the Exeter destroyer. When I get to it, there is already a squadron of Kilrathi unloading on the ship, and I can never get rid of them before it's destroyed. Anyone have any advice?

I always found the "defending a capital ship" missions to be a real pain. If I remember correctly, I think some of them are incredibly difficult to successfully defend (if not impossible). For some of those missions, I was only able to complete the mission by using cheats.

Since the first wing commander has a branching storyline (until you get to the last sector), I wouldn't get too stressed over the eliminated destroyer. Because of the branches, you can continue the game without saving this destroyer.

Taunt enemy ships ^_^
Use C to have communication screen, then select the enemy ship canal and taunt them (die furball...).

That's a tough mission because the enemies are Jalthis, the most firepower heavy fighters the Kilrathi have.

The best advice I can give you is this:

1. As others have said, taunt the fighters off the Exeter.

2. Go in full bore when you arrive and try to take out at least one fighter on your first pass. Save a missile for when the shields are down.

3. Your colleagues suggest that the Jalthi's armor is weakest in the rear of the fighter. They are correct about this. Plus, since WC1 did not have rear turrets, this negates much of the Jalthi's capability to ruin your day.

The Exeter's tough and can take more hits than the Drayman, which I am convinced is made of toilet paper, so if you can take out at least Jalthi fast, you should be set.

This mission is substantially less difficult though than attempting to save the Ralari in a later mission. I've managed to succeed there ONE TIME without cheating. I sometimes choose that mission to deliberately eject right at the start and get the Golden Sun if I want it.