Marche stands in the desert and remarks at how quiet it is. Then Llednar Twem walks in and tells Marche to go away, but a seam opens and he and Marche get drawn into it. This is the hall of the final crystal. Llednar orders you to stop, but Judgemaster Cid appears and uses a law card to forbid Llednar's most powerful move, Omega. Llednar engages you.
Engagement with Llednar Twem
Objective: Defeat Llednar
The goal of this mission is to defeat Llednar, but as you will discover, Llednar is immune to all attacks, taking 0 damage no matter what. But a quick way to end this battle, apparently, is to have Marche do a Fight attack against Llednar, who will then try to use Omega on Marche, and will subsequently get thrown in prison with a Red Card from Cid. Another way to fight this battle is to take out all of Llednar's allies (two Templars and two Titanias). Llednar will be sent to prison after the other enemies are gone.
With Llednar gone, Marche decides to fight the final Totema, Mateus, and destroy the final crystal so that he can go back home. When he approaches, Mateus transforms into Ritz, Doned, and Mewt, and Marche himself, all trying to convince Marche to leave the crystal alone. Marche, of course, refuses to stand down.
Engagement with Mateus
Objective: Defeat Mateus
Mateus reappears, surrounded by the apparitions of Ritz, Doned, Mewt, and Marche, who transform into Vampires. It is a good idea to include healers in your party. Be sure that most or all of your party has the ability to use a Phoenix Down or Life spell, because Mateus and the Vampires are powerful enough to kill several of your party in one turn. If you run out of allies with the ability to revive KO, you could find yourself in serious trouble.
Also, include people in your party who have very high attack power so you can do a lot of damage to Mateus in one turn; over 400 HP makes for a pretty tough opponent, but up against an attack like a Paladin's Holy Cross or a strong Air Render from a Fighter, you will quickly chip away at Mateus's health.
Note also that your starting squares to place your units are not in a very good place. There is a pit in front of your characters that will limit their movement a lot. I recommend placing your units near the back and to the sides so they can travel along the two sides of the room to the larger area behind the pit you start near (Mateus and the Vampires are likely to block the middle path unless you do so first).
The Vampires will keep coming back to life, but they are too damaging to just ignore, so try to take down a bunch of them at once with curing spells or life spells. There are undead-killing abilities like the White Monk's Exorcise and Archer's Burial that can completely remove the Undead from battle, preventing them from resurrecting, so you might want to consider those moves. They have a low success rate, however, so don't rely entirely on attempts at those spells. Also, blindness is a very helpful status ailment to inflict on all of the enemies in this battle, particularly on Mateus.
When Mateus is gone, the final crystal will shatter, and Marche will be happy that he will soon return to the real world. But to his surprise, the world does not change. Cid confirms that that was the last crystal, so he's confused, too.
Queen Remedi then appears, saying that the world will continue to exist as long as it is wanted. She says that Mewt still wants this world because he needs his mother, and she says that even Marche hasn't fully let go of this world.
So Marche realizes that he needs to have a discussion about this with Mewt. When the mission is over, go to any city and you will see a cut scene in a pub, where you overhear clan members talking about an event in the palace. Montblanc says that they are talking about Gift Day, officially known as Audience Day. This is a day when you can get an audience with Remedi and Mewt if you have connections, or if you bring a really impressive gift.
Montblanc says he will ask his brother Nono (a skilled machinist) to make a really powerful gift so that Marche can get an audience with Mewt. When the cut scene is over, you can place the Materiwood symbol on the map.