This is specifically for anyone who is having trouble unlocking all of the cars.
I had 17 cars unlocked, including the Hogsticker and Armadillo. One car wasn't unlocking preventing me from getting this achievement. I had played over 300 matches and unlocked every other item in the garage.
If you are having a similar problem where a car just absolutely won't unlock, the solution is to delete your save data (Delete from everywhere, not just the console) and start over.
I did this and then got the achievement after following Obscure Ale's guide.