This is pretty quick and easy providing you've unlocked a car that has spikes on the front of it.
The "They Don't Like It Up 'Em" Mission is for impaling 35 peds, so what I did was go to main menu...
* Choose Freeplay
* Choose Stadium Approach (can be done anywhere, but this one has loads of peds in and around the stadium)
* Choose Ped Chase (probably doesn't matter which type you pick here. I chose unlimited points and game length).
to change car to something with spikes on the front. (I chose Degoryun)
* Click Start.
* Drive inside the stadium and around the front to skewer a ton of peds. When the amount of peds gets scarce, just press
-> RESTART EVENT and it'll start with loads more peds for you.
This has tracking, so you can snap achievements to see your progress, or use the xbox app. It shouldn't take more than about 10 minutes.