This requires a lot of zombie kills and object destruction. Have a machine gun that is upgraded fully, and a flamethrower for use in narrow spaces that contain boxes and other objects. Keep riding and shooting to get a +30 seconds bonus. Killing zombies will result in more time, but there are many more objects to destroy. The most efficient way to maintain a running combo is to keep driving while shooting the machine gun. "Kill and destroy" will be earned when the combo is converted into extra time, which happens when you stop destroying and killing, or when you run out of time.