This sounds really high at first until you start getting better times on the bosses. Better boss times and higher ratings equal higher bonus points at the end of the level. To get this achievement you have to be able to get at least an A Rating on all of the bosses. Getting an A Rating will net you close to 2,000,000 points by the end, getting an S Rating will net you somewhere over 3,000,000 points. Grabbing lots of power ups, medals, and working the multiplier will get you over 4,000,000 points. Again, as far as Single Player or Co-op is concerned for this achievement, it seems as if only the host gets this achievement at the end of the game if you are playing online. So either grab this in Single Player or host your own Xbox Live Co-op match.