The Freeze Gun costs $440 and can be purchased from the Garage in the Weapons section.
Purchase the Freeze Gun and equip it to one of your slots (up, down, left or right).
I would recommend fully upgrading the Freeze Gun as it takes quite a while to freeze an opponent with a weaker weapon.
Freeze Gun - $440 > $440 > $480 > $520 > $800
To get this achievement you should let your opponents overtake you so you can shoot them from behind. This achievement is cumulative across races.
Please comment if you vote negatively. I will edit my solution to make it better.