First off you have to have 2 xbox profiles. Start GH3 and sign into the profile you dont want to beat the boss on. Go to cheat menu and turn on "No Fail" and "Easy Expert". Back out to main menu. Go to quick play, then choose "Expert" and then choose "Slow Ride". Play like the 1st 10 notes, when you get to the long orange note, click start and go to main menu. Sign out, then sign into your profile you want to beat the boss on. Then go straight to career and go to the boss battle. The computer will miss all his notes and lose within a few seconds.