Here are two pretty quick and easy achievements for Battlefield 3. You're welcome, by the way. To get the story-specific 'Not On My Watch' achievement, load up the level "Operation Swordbreaker". All you do is provide covering fire so that your buddies can get out safely. Do this, and you'll net 20G on Xbox 360, and a bronze trophy on PlayStation 3.
Now, to get the 'Roadkill' achievement, you'll have to be on the level "Uprising". All you do is sneak over to where there's a car hanging on to a ledge by just a thread. Knock it down to crush some guards, and you'll get 20G, or a bronze trophy.