Get "Roadkill" early
This achievement can be gained early in the game in "Charlie Don't Surf". Past the main highway (where a turret-mounted vehicle would attack), there are about three cars in the narrow street. Wait for the enemies to take cover behind the cars and shoot the hell out of those. That's how I got it.
This achievement guide suggests "Death from Above". I'm not very sure about it but I won't be able to prove or disprove it either (as I already have the achievement). Would the enemies killed be counted toward the car explosion as you wish or just your missiles? To play safe, I suggest at least try getting the achievement in "Charlie Don't Surf" beforehand.
I have preferred gaming PCs over consoles since 2012.