There was a video on youtube which told u to make 2 lines with 5 cars in each line right outside your safehouse in bohan then dial for 2 fire trucks and an ambulance. Line then up against your cars and stand on top of the ambulance and throw grenades into the cars.
Every time the cops are on u just save the game and your cars will automatically be saved. And if u fail u will just have to load up and try again. Thats how I got the achievement