After the many theories, I decided to test them all out myself.
Here's the REAL way to earn the "Vehicular Slaughter" achievement:
- Why not make it easy on yourself? Set difficulty to Recruit.
- Do not worry about any vehicles before the tear gas scene.
- The achievement begins at the motorcycle escape finale.
- Take out all motorcycles and the one truck before crossing the river.
- Once you make it onto the truck with the MG, be sure to take out the one helicopter, all motorcycles and moving trucks. Do not worry about the stationary trucks parked on the side of the road.
- Achievement unlocks mid-jump to train.