Instructions
Complete the Prologue. This serves as the tutorial for "Dissidia: Final Fantasy." You must complete it before you can access most of the game modes.
Complete the Destiny Odyssey story mode with any character. The numbering system for Destiny Odyssey does not indicate the difficulty of this mode for the character. It indicates what "Final Fantasy" game that character appeared in. The two easiest Destiny Odyssey modes are Destiny Odyssey IV, Cecil Harvey, and Destiny Odyssey VII, Cloud Strife. Completing any Destiny Odyssey mode unlocks Shade Impulse.
Complete Shade Impulse mode with any character. The final battle of this mode is against Chaos. This is a three part battle. This battle is easier for faster characters, who can dodge or counter Chaos' attacks more easily, but can be completed with any character. The strategy required varies greatly based on what character you have and how he is equipped.
Open the PP Catalog in the menu after defeating Chaos in Shade Impulse mode.
Purchase "Chaos in Quick Battle" from the PP Catalog to unlock Chaos. This option costs 100 PP.