After meeting up with your father, you'll receive the Ganglari, a magic-infused sword. He'll then order you to test it on the Hoshidan prisoners. Rinkah and Kaze will the be the named Hoshidan characters you need to face and on your side you'll have Gunter and Felicia (male avatar) or Jakob (female avatar).
To start off, step on the Dragon Vein and activate it to clear the rubble in the middle of the field and reveal a large healing tile. You don't need to intercept the enemies; just position all your characters inside the healing circle and let the enemies approach you. Make sure to keep them close together so they can provide support to each other.
Felicia/Jakob can provide healing and can do ranged attacks that can pass through walls. Gunter is a great knight that works well as a tank in this battle so you can have him take the brunt of the damage. This is a very easy battle as long as you're fighting within the healing circle. Once all enemies have been routed, the mission is complete.
After defeating all enemies, there will a short scene where your character will prevent the prisoners' execution out of mercy. Your siblings will step in and save you from your father's wrath. The following cutscene will reveal how your kindness granted the prisoners their freedom and lives.