Finally I beat the hard badge! The trick is to try and use the bottom of your car to smash the zombies at the start, saving your ammo on the gun, then use the boost at the end to smash through the boxes :)
An easy way to get the hard badge is to assign the "Star power" or whatever button, to a note key. That way when you get star power, you can activate it when you hit the next note.
some keyboards won't do certain key combinations, you can use 'W' for boost instead (this is mentioned in the options panel of the instructions : )