ok i can help you (i think)
do not destroy the shield generator. quickly wrap the first at-at you see and the one by the shield generator, then destroy those turbolasers by the cliff, and then just circle around in the canyon (making sure you don't spot the at-at on that can destroy the atpts, he won't destroy them if you don't see the at-pts), and then when the gate is down, go over near the weapons factory, and get ready to blast all the tank droids, usually you can get them all in 2 passes, once they're gone, fire like mad at the ones on the other canyon, and then destroy the 3 up by the corner near the factory, then get ready to shoot down the tie bombers, it's alot easier if you stay near where they come flying in at, as you can destroy them fast, then when the first wave of bombers is destroyed, go over and help the atpts destroy the last 3 tank droids near the landing zone, and then quickly turn around and destroy the rest of the bombers.
Then just take down the shield generator, and there should be some tank droids around that area, so get some extra kills off of them, and strafe the snowtroopers, then go get the bonus, destroy those 2 missile turrets and the 2 turbolasers near the top, destroy the barracks that's holding the bonus, then go get some more accuracy by shooting the at-at down where the atpts started, and then turn around and destroy the weapons factory and hopefully get the gold medal.
I hope i helped.