Ok, to start off, look at what you have to do here. You have to
1.Shoot apart the walker thats shooting at you when it first starts.
2.Keep the prisioners alive.
3.Wipe out the walker and stormtroopers at the top who are shooting at the prisioners.
By the way, the prisioners don't start dieing till you go up the elevator. The first thing on your list is to get the walker under control for the time being. You need to run down to those big doors behind the walker and get him to follow you. Once he's about 3/4 the way down the canyon, run like heck back up, hugging the right wall. The walker well get stuck there and out of your hair for the time being. Now go up the elevator and proceed to shoot the crap outa the stormies and the walker, and then turn around and you'll see a little bit of the walker, mow him down without worry of getting hit. Hope that helps.