The most foolproof way I've found to do this is to go to the top of a building and find a 2 x 2 grid of air conditioning fans. Then start bouncing on top of them, move the right stick so that you're looking straight down, and just move the circle between the squares on the grid. Every time the center of the circle crosses a line between bounces you will add to your combo counter.
Unlike jumping between two cars there is almost no chance that you will miss and hit the ground between items.
The achievement will not actually unlock until you hit the ground and break the combo. Just make sure the counter on the right side of the screen is > 100 and then touch the ground. Pop!