This is easily done BEFORE you pick up Genki as the event won't start itself. This allows you to drive around at your own pace within the mini game.
Drive over pedestrians to obtain your fire weapon. Genki's voice will call out and tell you when you have it. Activate it with
and carefully burn pedestrians with it. I found that if you drive around quickly it doesn't seem to count. (Coincidence?) Whether they go out of range or die from another cause I don't know. I got it by slowly driving by and making sure fire hit and not my car.
This achievement will pop as soon as you hit 50, allowing you to ditch the activity once obtained.