The story continues in Alexandria Castle. Steiner and the Alexandrian Soldiers will be ready and awaiting Princess Garnet’s orders. You will be prompted to give orders to each of the individual soldiers - the clues to what each of the soldiers are good at were provided when you found them all during the Knights of Pluto side quest early in Disc 1.
The correct assignments are as follows:
Beatrix will reward you with a different item depending on how many of the assignments you got correct. You will receive the following:
Hi-Potion
Elixir
Angel Earrings
The game continues with you in control of Steiner and Beatrix in Alexandria Square. Beatrix is only temporarily in your party so do not equip her with anything important - least of all, your newly acquired Angel Earrings.
In fact, you should steal her 
Cross Helm by de-equipping it and replacing it with something that is not as strong and that you can easily replace.
Steiner, on the other hand, should be fully equipped with the latest and greatest equipment as it has been a while since you have had him in your party. Equip him with the 
Coral Sword that you purchased and with the Cross Helm that Beatrix was wearing.
Beatrix and Steiner are going to have to do battle against several Mistodons. They are not particularly strong - a few regular attacks should kill them.
Kill the first Mistodon and travel southward into the next area of town. Remember that Mistodons are undead creatures which means that healing spells, specifically Beatrix’s 
Life spell, will instantly kill them. There are approximately 7 individual battles to fight as the story continues. Keep moving south.
After defeating the 7 Mistodons, the game will return to Dagger in the hallway of Alexandria Castle. None of the doors are open on the lower level so you should travel back up the stairs to the second level balcony. Go through the doorway on the bottom-left side of the screen, up the set of curving stairs and back towards Queen Brahne’s old room.
There is a barrier guarding the pathway to the room, but take the stairway and door on the left side. The knights will lower their swords blocking the pathway back down as you ascend the stairs.
Take the newly formed stairway to the top of the Altar. There are a few more cutscenes involving Princess Garnet and the rest of the crew who are onboard the Hilde Guard 2 with Reagent Cid.
Eiko will leap off the bow of the ship and down to the castle to meet up with the Princess. Together, the two will summon the Holy Guardian, Alexander, to defend the city against Bahamut. This is the first time you will be introduced to another of the primary villains in the story - Garland.
You regain control of Zidane, with Vivi, Freya and Amarant, in the main hall of Alexandria Castle. Take the time to equip each of them with any new weapons or armor that you have picked up. It is time to take care of an optional boss called Tantarian.
This is the last time that you will have an opportunity to face Tantarian. Defeating it yields a very strong accessory: a pair of 
Running Shoes which grant a character the Auto-Haste ability.
In preparation for the fight, equip your characters with 
Devil Killer (as the boss is Devil-type), Antibody to protect against poison attacks and the Counter ability if any of your characters have it.
It can also be helpful to place each of your characters in the back row to reduce the amount of damage they take. This will impact their attack power, but most of your damage will be coming from Vivi’s spell casts anyway.
Go back up to the second floor and into the Guardhouse on the right side of the screen to save your game by speaking to Mosh the Moogle. Travel back down to the main floor of the Castle and go through the doorway on the left to find the Library.
The action button appears near the bookshelf near the bottom-left side of the screen. Choose to listen and then accept the challenge from the monster.
Level: 41
HP: 21,997
Tantarian is a very unique boss fight. It can use the following attacks on you:
Unfortunately there is no way to remove the “Doom” status effect which means that your characters will likely die a few times during this fight.
Tantarian is actually a boss inside the book that you are fighting. Each time you attack the book it will turn a page until you reach pages 150 to 200 which will reveal the actual boss that you are fighting. It will automatically close and flip to a new page if you attack it with a physical attack in this state.
Have Zidane steal for the duration of the fight in the hopes that you get extremely lucky during the battle and end up walking away with the 
Demon’s Mail. Just remember though that obtaining the Demon’s Mail is extremely rare, and while it is certainly a fantastic item to have, it is not anything worth getting held up on. Use Bio on the true Tantarian once it reveals itself to increase the amount of damage done.
The trick is to get Tantarian into the 150 to 200 page mark and then strictly focus on magic-based attacks while it is revealed so that you can keep damaging it directly. Avoid all physical attacks by having Amarant and Freya focus on healing (Reis’s Wind works very well), have Zidane focus on stealing, and have Vivi focus on casting Bios.
Steal:
Ether (common)
Elixir (uncommon)
Silver Fork (rare)
Demon’s Mail (very rare)
Drops:
Running Shoes
Follow the exact same pathway that Dagger took to get up to the Altar. The game continues back in Lindblum Grand Castle after a number of cutscenes.
