Genesis is a boss in Crisis Core -Final Fantasy VII- fought in the abandoned Mako Excavation facility near Modeoheim. He can also be fought in Mission 5-1-5, although the awards for beating him are different.
Genesis's special attack is Black Flurry which inflicts moderate damage and Curse status. Genesis uses Regen and can use Defense and Magical Defense to become invulnerable to either physical or magical attacks, and can even use them both at once, which makes the player wait for the effects to cease before being able to damage him. Genesis also uses Osmose to drain Zack of MP.
The player should start by casting Regen and Barrier on Zack. The Curse status Genesis inflicts can can be cured with Remedies or protected against with accessories. With high ATK and VIT, Genesis can be defeated quite easily.