OK, I figured it out, but can I explain it.
-when you fly out of the hanger, get to a comfortalbe altitude, and head toward the AT-AT's. Then press R2, this will swith to co-pilot control. Move RT anolog, so that in co-pilot view you are viewing your ship from the right side as far back as you can go. Your just changing the perspective of the view. Then switch back to pilot by pressing R2 again. Now head toward the AT-AT and pass slow and low on the right side, use rt anolog to control speed. This is where it gets tricky, you are gonna need to be quick here. As you approach switch to co-pilot again, and then press R1, this will fire the harpoon. Try to time it so you hit a leg,(you will notice that because you set up the right perspective you get a nice look at the harpoon. Hopefully you hit a leg, then you need to quickly press R2 again , which takes you back to pilot, and immediatly start banking hard to the left. It will take a few tries to get the hang of it and dont be discoraged if you dont get a hit right away. Once you get it then it's just a matter of flying a tight circle to bring the snowwalker down. ( it will happen automatically so no need to drop the cable. Best of luck. No way Im saying the 'force' line!