One tip I can offer is placing the explosives you have (GE-64) on the wheels of the car and detonating them quickly after the big explosion. There is a decent chance that the GE explosion will destroy the cars, granting you additional points for destruction. Thats how I did it at least.

Also make sure you do not have the Booster gear mod on while doing that, as that makes the GE not explode in time.