You don't have to take down the "walkers" that are moving, just the stationary ones being built. You will need to take out every thing at each yard,.... buildings, at-at's, at-pt's, troops. Kill every thing until you see no more red dots in your HUD.
Then on your way to the next yard go by the tall platforms that the Tie's are parked on and wipe them all out for extra kills , then procede to the next yard. Don't forget the bonus, it's in one of the round bunkers behind a platform. When you come to the next yard do the same as you did at the first. Wipe out every thing.
I usually would take out missle launchers first and then gun turrets so you can work on the buildings with out to much trouble. You shouldn't need to chase down any Tie's that might be flying around, but if one is in front of you give him a quick blast but don't waste time trying to kill him if it's not convenient.
There is only two yards, (I think, if I remember correctly) and I usually just go strait at the turrets. If you find you just can't make it with only 3 lives, just use the code....iamdolly....for infinite lives.
Hope this helped. GUNNER
"With each passing moment,you make yourself more my sevant."