On the way to Deti Plains, you are attacked by monsters. Beastmasters and Hunters are good choices for units to bring along because of their monster-specific abilities.
The battle takes place on some cliffs, so Galmia Shoes would be a great idea because it lets you jump any height, giving you quite a tactical advantage as you can have a unit attack the enemies from behind.
The Coeurl can petrify your units, so that one is a pretty big priority to take down soon.
The Jawbreaker has LV3 Def-less and LV5 Death, both of which will 100% of the time hit any of your units who have a level divisible by 3 or 5, respectively. If none of your units meet these criteria, you're safe, but be careful if they do or if they level up and become vulnerable to one of those attacks as a result.
The Big Malboro and Lilith are dangerous because of their status-affecting abilities. A Skill law would be a great idea for this battle. The Lilith is particularly dangerous because of her Twister ability, which will cut a unit's HP by half. This is an area attack, so avoid letting your units get clustered together.
The Thundrake does a lot of physical damage, so keep your distance and take it down quickly.
Once the enemies are gone, Cid arrives and Marche asks him if he's met with Mewt and the Queen yet. Cid asks Marche to go to Siena Gorge to look for amber for him so that he and Ezel can create an anti-law, even though Cid will be doing so at great personal risk.
As Marche and Cid leave, Ritz and Shara are visible. Shara asks if Ritz wants to follow Marche to Siena Gorge. Ritz doesn't answer.