After the alluring scene, you'll have to face your steamy big sister, Camilla. There are several dragon veins in the are that will instantly drop the HP of the enemy units in the area to 1HP on specific areas. Camilla will trigger the nearest dragon vein in her location if one of your units approach the western bridge. Make sure to check the spots where dragon veins will target once activated to avoid getting caught in one.
There are several houses that you can interact with so make sure to check them all out individually to get free items. (Spirit Dust, Heart Seal, Talisman, 10000G, Wane Festal)
Send one of your noble units to the southeastern corner of the map to be able to activate the dragon vein there which will target the treacherous Hans's group in the eastern bridge. Hans is quite dangerous if his attacks connect so make sure you have some healing items ready.
As you approach the bridge, be careful of Beruka and Camilla's range. They're flying units so avoid getting prematurely targeted without proper support. Your archers will make quick work of them as long as they don't get attacked first. Camilla's magic is high-tier so make sure to save your game mid-battle if you're playing in Classic mode.
After routing all enemies, read through more dialogues. A mysterious knight, Scarlet and her wyvern riders will arrive on the scene to drive the remaining Nohrian forces away. After the battle, Scarlet and Ryoma will join your ranks.