You are offered one, and only one, chance to fight against the super-tough Tantarian. It is available when Zidane and party must enter Alexandria Castle to look for Dagger and Eiko (beginning of Disc 3). On the way up, enter the left Library room, and on top of the shelf at the bottom, you can challenge the Tantarian. If you do not take this opportunity now, it is gone forever -- after this segment, this portion of the Castle is closed forever.

The Tantarian, essentially, is a book. The brain behind the book is a round bluish creature that inhabits one (or more) of the pages in this book. In between attacks, it will open to a page between 1 and 200, whose number is equal to the amount of damage you have dealt to it. (If you attack Tantarian while the book is closed, you do very little damage. The book offers a lot of defense.) If you do above 200 damage, it resets. Most of the time the page is blank -- every once in a while, though, the page where the creature is hiding opens up. It is at this time that its defense is lowered and you're able to do the bulk of your damage. A single attack, though, will close the book again, and you will simply have to wait for the right page to open again.