Cars explode very easily in this game and have a fair kill radius for enemies. You can get this by waiting for enemies to take cover behind cars (happens often) and then blow them up. You should easily get this over the campaign.
There are also sections where reinforcements roll up in a car. Shoot it until it is on fire, wait for them to get out (I waited for the doors to close after they got out) then blow it up for 2-4 easy kills.
An excellent spot for 6-8 kills instantly is on the last level when chasing Alvarez, he runs through a small garage with a few cars and a bunch of guys. Blowing up one car almost always causes a chain reaction with the others and wipes out almost everyone in the room.