Use the six-bar ATB weapon on Noel, give him Bravery and Enthunder, get Long Gui to 999%, use his special attack. It also says to give him 4x Delicate Iron Bangles and to ensure his health is green (to ensure he gets even more bonus damage). It's pretty much making Noel as powerful as he can possibly be, I guess, and attacking an enemy you can easily get to 999% without killing it first. I'm sure there are other ways you can do it, though. If I remember right, it mentions you don't have to max out his crystarium when doing it that way, but I can't remember for sure (I'm at work at the mo').