Quite simple really.
On every level there are different Monster Obstacles for you to break though ranging from a Alien Spacecraft to Boats.
Overall there are 20 thought out all levels with roughly 2 per track. But all you have to do is break though 8 different ones.
Here is a list of each one and their locations on the track:
http://www.xbox360achievements.org/forum/showthread.php?t=80...Credit goes to danteraines452.