Ritz stands in the way of Marche's progress to Ambervale. You'll have to fight her and her clan to continue on.
Because of the narrow space in which you have to fight, don't get too close to that bridge, or you'll all be struggling to fight each other. Stay back and Ritz's clan will come to you, into the area with a larger fighting space.
The Assassin has Last Breath which can kill a unit in one blow.
The Summoner has the very potent Madeen spell. Keep your units apart to avoid letting them all get hit.
The Elementalist will cause status ailments.
The Sniper is very dangerous with Doom abilities, and the ability to destroy your weapons and armor.
Ritz is the main objective. Since she's right at the front of the group, you can probably avoid having to take down all of the other enemies. Note: the enemies have some good weapons, so you might want to invest some time in stealing them. Ritz is a Red Mage, and she has Doublecast, so she will do a lot of magic damage to you. She can also put you to sleep and then cast an offensive magic spell. She can't be silenced. A Color Magic law will stop her from casting magic.
Once she is defeated, she has no choice but to let you through to Ambervale. Ritz refuses to join Marche because her mind is still made up to stay here. Marche asks if Ritz will hate him if he brings the real world back, to which she replies that she won't. She doesn't think that Marche is doing anything wrong, but she wants to stay there. Marche leaves. Ritz is sure that Marche will be able to bring the real world back because he's changed from the timid new kid that he was back in St. Ivalice. Ritz seems envious, because she feels that she has not changed at all. She and Shara leave.