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!