Wrecked 30 or more cars in CITY ESCAPE Act 2.
Pretty easy achievement. Right from the start, stick to either side of the road as you ride down the hill. Take it slow and try to tell which side has more cars grouped together and aim for them. You don't have to hit them very hard to wreck them. After you lose the board, you'll do a bit of platforming, and eventually come to a part where you are just running down the streets. Again, just stick to the sides and look for big groups of cars. To wreck them here, either boost through them or just jump on them. I haven't taken the time to count how many cars I had hit, but as long as you don't ride in the middle too long and miss a bunch, you should be fine. If you want to be absolutely sure, after the big truck chase, when you are flying towards the goal ring, aim to the right or left of it. There are 6 or 7 cars just sitting in the little area near the goal.
As soon as you hit the goal, the achievement will unlock, so don't panic if it doesn't show up before then.
EDIT: Added video. Credit to KLachievements for making it