Crisis Core Help!

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

I'll type the strategy guide instructions here:
The Vajradhara Karura is a tougher foe than the Vajradhara Indra, plus this beat packs an attack that can deplete Zack of all his HP (provided he has yet to break the 9999 limit). Equip Zack with a Phoenix Down and try to stay behind the beast at all times. The standard sword attack can inflict significant damage from the rear, so hack away! Switch to magic -based attacks when farther away or on the run.

Vajradhara Karura has 84,658 Hp! Good luck! :D