You will most likely have hit 250 cones on your casual playthrough of the story - if not, you're likely to be close. I was at 244 cones.
To check your progress, visit the orange mechanic and hit
to see your abilities - and the cone one does show how many you've hit already.
Once you unlock the ability, go to the orange mechanic (you'll spawn at one if you blow up, fastest possible way to get there) and activate it in the menu.
Shoot 12 pedestrians with the cones you now can shoot out of the trunk with
an the trophy will unlock.