When the player attacks Ralvuimago physically, it will become compact by curling into a ball, which raises its defenses and causes it to counter all physical attacks with Power of the Land, which deals Earth damage to all targets, and heals the boss. Ultra Sound Wave inflicts Mini status upon one character, who is then vulnerable to Ralvuimago's physical Stab attack.
Zidane should try to steal the Oak Staff as it will be Vivi's earliest access to the Bio spell (if one did not get it from the Chocograph yet). When compact, Ralvuimago will not attack except as a counter. This can make the battle easy, as all the player has to do is use a physical attack with Zidane to make the boss compact, then wait until it reverts and use another physical attack with Zidane.