Send Han over the AT-AT, shoot the iron triangular block supporting the hanging snowtrooper in the top-right corner. The laser blasts should ricochet off a few angles and take out the storm trooper next to the TNT. With a little luck, the TNT will detonate and cause the AT-AT to fall to its knees. If the TNT is not detonated with laser blasts, it can still detonate by the tower with TNT fall to the left.
See Video Walkthrough.