Send Han towards the nearest chain, shooting laser blasts to the furthest chain, let the second pile of wheels hit the TNT and destroy most of the level. Send Leia to finish the level.
This is the better strategy. Send Han above everything, shooting out the chain on the right side of the level. The pile of iron wheels should destroy the tower below it, and Han should sever the small chain in the middle of the level. The released wheel should act as a wrecking ball, destroying all structures below it.
See Video Walkthrough.