This can be challenging and fun. Takes about an hour. Make sure you are a high level, the Real Mega Buster makes it a lot easier. Basically you will have to kill everyone other than the 8 the guide tells you too. Enjoyhttp://www.gamefaqs.com/console/xbox360/file/928326/46838
NOTE: I did not write the guide, I'm just informing this site about it, let the author know of their good work. Thanks.