To unlock Giant Attack 666 Mode, you must beat arcade mode with Neko-Arc Chaos while doing his special story mode (Hold Button D while selecting Neko-Arc Chaos till the story mode sequence opens. If Neko-Arc Chaos starts talking about something referring to HD DVD and Kohaku appears, you've got the right story arc. Your mid-boss will be 4 Neko-Arc Chaos & final boss will be Neko Chaos Black G666 instead of Satsuki & White Ren).Giant Attack 666 Mode allows you to fight against Neko Chaos Black G666 (aka: Giant Neko-Arc Chaos Robo) with any character to see how fast you can beat him.Please note, this mode requires you to have the Version 1.01 or 1.02 patch to unlock this mode (problems in the game did not allow it to be unlocked with the initial release).