Not too hard once you know the trick.
For each item you hit, your earn a little money and increase your multiplier for the next thing you hit. When no damage is caused for some seconds, the multiplier will return to zero.
Search the neighborhood for some fences (or terraces with chairs and tables) close to each other and drive from fence to fence (or terrace to terrace or a combination, you get the idea) while slaloming through them. This allows you to increase the multiplier and keep it, since the first fence has probably reappeared while you were knocking down the other fences. The slaloming helps to maintain your speed, so you don't stall and lose your precious multiplier.
Once the multiplier is around 100x, every piece of fence you hit will give you loads of money, so keep going a little longer to complete the level. Completing any of the levels should be easy once you find a good spot to drive circles while constantly hitting stuff (some of my favorite spots: the fence around the airport, the terrace around the insurance fraud building in Sommerset and the sidewalk around the north east block in the museum district).
When there are no fences around (museum district), it also works with light poles and fire hydrants. It only takes a little longer.