This achievement glitched for me... in a good way.
I cleared my cache for other reasons and the very next time I started up L4D, this achievement unlocked along with 20 Car Pileup (both popped on main menu, before pressing start).
I had previously played L4D2 and still had a guest signed in, only have the Crash Course DLC.
I previously only had about 5 or 6 smoker kills and absolutely nothing towards the 20 cars.
Let me know if it does/doesn't work and if you have a guest or not, and what DLC you have :)
EDIT: after I cleared the cache it said I didn't have the Crash Course or Survival Pack DLCs. I thought I'd try clearing the cache again to see if any more achievements popped (they didn't) and the next time I started L4D it said I had the DLC again. So instead of re-downloading any DLC, just clear the cache again.
And no, this did not affect my progress towards other achievements (such as Zombie Genocidest).