You are ordered to capture Notre Sagesse. To get there, you must pass through Fort Dragonfall.
Prepare Before the Battle
Most of the enemies here use lances, so you should bring mostly units that use axes and bows. There are also a fair number of archers, so you can bring some units that use swords and magic to deal with those. In addition, there are a few sword-wielding enemies, so you can bring a unit or two that can use lances and hidden weapons (i.e. shuriken). Silas is a good choice.
Azura is automatically in this battle.
There area couple of chests, so bring a Ninja so you can unlock them. Azura already has a chest key, so she can unlock one of the chests herself.
You don't need to defeat all of the enemy units in this battle. You just need to reach the boss unit and seize the tile that he is guarding.
First, have Corrin go talk to the ally unit, Nyx, to recruit her into your ranks. She is a magic user, so she should attack the archers, but she is vulnerable to the many lance-wielding enemies here.
Some of the walls here can be broken by attacking them. Use this to your advantage to reach the enemy more quickly.
There is a Dragon Vein in the northeast corner, near where Azura begins the battle. She can activate the Dragon Vein. This particular Dragon Vein will drop units' HP to 1 if they are standing in the affected area (including your own units). Wait for multiple enemy units to be standing in that area, then have Azura activate the Dragon Vein.
Otherwise, keep an eye on the enemy's attack range, and keep Azura away from it until you can send more units closer to her. A breakable wall near Nyx will help you reach Azura more quickly.