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Talk to Kotomine on 1F.
Talk to Sakura in the Nurse's Office to receive your final weekly item.
If this is a New Game +, check the table where Taiga used to be on 1F. This will only appear if you have completed all of her quests and causes an optional boss to appear in the Arena.
Talk to Kotomine at the Arena entrance and enter the Arena.
You can leave the Arena and return as much as you want with no penalty.
This Arena is just a straight hall with one large room. It contains three enemy types, Claustrophobia and Nephilim:Δ from the last week, and one new enemy, Babel's Piece.
These Nephilim respawn. They give by far the best experience and, more importantly, drop Elixers. The fastest way to farm them is to defeat them then leave the Arena to force them to respawn immediately.
If you're playing a New Game + and unlocked it already, you can challenge an optional boss by heading to the north exit and choosing to keep going. But, of course, if you don't want to fight it, you can always leave from the entrance.
Equip the Exorcist Blade and bring a lot of Elixers. You should rely on the Elixers for all HP and MP recovery, so use the other Formal Wear slot on something more directly useful than recovery.
Monster always uses Twin Towers for her first action of the first turn. This attack deals a lot of damage, so prevent or defend against it. She will use this Skill again for the first action every 10 turns. Once she has low health, she will start using it for her 6th action.
She has four other unblockable attack Skills that ignore Guard and have a chance to seal one of your actions between Attack, Guard, Break, or Skill. If you use the matching action when she uses one of those Skills, it will be sealed. Otherwise, you still take magic damage. This can make spamming attack Skills to cover slots you can't see risky since all 3 Servants you could be using will be in trouble without Skills. However, she will never use more than one Skill per turn, and she only uses sealing Skills every 3 turns. The sealing effects are permanent, unlike typical status ailments, and cannot be cured. The only ways you can avoid it are with Archer's 7 Rings of Heaven, Hack, or Caster's Skills that can stun on the following action. And that last one is not a guarantee.
If your Attack, Guard, and Break are all sealed, Monster will use her Noble Phantasm. This is an utterly unavoidable guaranteed instant kill. Not even Saber's Thrice-Setting Sun can save you.
The best way to fight her is to go all-in on Skill spam and use up all of those Elixers you've been saving to keep it going. She has a ton of HP, so if you hold back and try to conserve those Elixers, you will be in for a very long battle, which is the last thing you want. Take her down hard and fast before she has many chances to get lucky with those sealing Skills. The best defense is definitely a good offense when it comes to this fight. Her attack patterns are relatively easy to read, and her basic attacks aren't all that strong. The most important thing is to keep your worries in check. Use Hack to stop any Skill you can see to be on the safe side, keep your MP up, and be aggressive, and you will be fine.
When you're satisfied with your grinding, prepare for the end. In preparation for New Game +, go to the Commissary and buy any Formal Wears you think you might have a use for in the early game—you keep all Formal Wear aside from the Scarlet Formal Wear while your level is reset to 1. Your consumable items do not carry over, but your money does. Sell any of those you aren't planning on using for the final battle.
When you've finished preparing, talk to Rin/Rani on 1F.
All you need to do is run straight until...
Let me say this to start: This is a much easier fight than Saber.
Savior will use his Noble Phantasm after 14 turns—an instant Game Over—but you should have this finished long before that.
He often starts turns by buffing one of his 3 standard actions. Madhyama-pratipad causes his Attack to absorb HP, Samadhi causes his Guard to absorb MP, and Dukkha causes his Break to stun. None of those are fun. Be extra careful of those when he applies one of them, or use Vanish Add to remove them.
Savior uses Sunyata for his 1st or 2nd action, which deals magic damage. This attack isn't that big a deal for Caster. However, as the battle progresses and more of the nodes behind Savior light up, its uses start to spread to the right, no longer constrained to the first 2 moves.
Dukkha Samudaya—used for his 3rd action—is quite a bit more dangerous. It nullifies any action you take except for Guard and counters for significant damage. On the other hand, if you use Guard, it doesn't do anything. As more of the nodes behind Savior light up, the actions where he uses this start to spread to the right through the turn.
Twice will sometimes use a Code that seals your Skills. The only ways to heal this seal are with Sakura's weekly lunches or the Scarlet Formal Wear.
His patterns are incredibly predictable—by far more than any other boss. Just be aggressive and give him everything you've got. Don't be afraid to spam your Skills constantly and use Elixirs to recover your MP. You don't get to keep those, so if you don't use them now, what are you saving them for?
Congratulations, enjoy the ending!
After the credits, you are prompted to save a Clear Data file. You can load from this file to start a new game that carries over some things from your previous playthrough. You keep your accumulated info for regular enemies, money, and Formal Wears—except for the Scarlet Formal Wear. Your consumable items do not carry over, and your level starts at 1.
A New Game + skips past the Prologue up to the part where you select your name, gender, and Servant. Changing those is the most obvious reason you might want to try this. You can choose another Servant and side with a different person if you wish, allowing you to experience different sides of the story.