Crisis Core Help!

[Carl the Llama]

Well ok im playing Crisis Core (fantastic game) and I am a little bit stuck on a mission: To End the War with Wutai: The Remnants Strike Again: Death Wish for Zack, the thing im stuch on? The boss hits me for 9999 damage (my exact health) even when I have mastered Wall and 105 VIT so in shourt my question is this: how the hell do I not take 9999 damage? how much vit do I need to stay alive?

P.S. I have just started a New Game+ so I only have 4 materia slots and 2 accesory slots.

EDIT: I just beat him, had to equip 2 Shinra Beata's not to take max damage 165 VIT and I still took 9998 damage lol, the my old friend Sephiroth decided to dish out a level 5 Octaslash on his ass xD