This should be very easy to get on the first level (The Battle For Endor). In addition to all of the Stormtroopers you'll defeat right at the beginning (12 are required to advance), you'll get an AT-ST a little later. Stormtroopers have a bad tendency to run under your feet. Just keep stomping around and shooting them. You should get this by the time you get to the end of the first section.