Roundabout

To unlock this power-up you need to kill 2,500 pedestrians. It's not too bad of a grind and you will likely get at least half way there during your main playthrough. After completing a couple speed runs, I was up to about 1,700 pedestrian kills. To complete this, I went to some areas where people respawn easily and I could run them over without needing to worry about obstacles much.

My suggestion for where to go:

The Beach in Area 2. This area is the southern/south-eastern/eastern portion of the second map area. Going along the beach areas will respawn the people before you come back and you can 20-30 per portion of the beach. The only real obstacles are lifeguard stations. The water is there and the people will get relatively close, but if you equip the Inflataboat, this won't be a problem. Therefore, I would suggest completing this achievement first if you're concerned about people entering the water:



Some guides for getting the 48x and 35x multiplier achievements recommend using the Beary Nice Music Box as well since running over people helps maintain the multiplier.