After joining Azura at Fort Dragonfall, you reach the port town of Dia, and the Hoshidan army attacks.
Preparations Before the Battle
The objective is simply to survive for 11 turns, so as you can imagine, this will be a difficult fight. Be sure to give everybody some Vulnerary or Concoctions.
Bring at least two archers and one magic user, because they can use the turrets to deal damage to multiple enemies at once.
The majority of the enemies here are lancers and hidden weapon users, so bring many axe and bow-wielding units. There will also be a fair number of club-wielding enemies, and some bow-wielders, so bring some sword users and magic users.
Defend Green Zone for 11 Turns
There are four green tiles at the top of the map. If any enemy stands on any of these tiles, you will lose. You need to prevent the enemies from reaching those tiles for 11 turns.
At the start of the battle, you will want to rush to the Ballistas and the Fire Orb with your archers and magic user, to prevent the enemy from taking control of those turrets.
At first, you can block the enemy's approach from the south by standing in front of the breakable wall to the left of the southern turret and the narrow passage to the right of the southern turret. You can block their approach from the west with one unit on the bridge near the Fire Orb. To block the enemy coming from the east, have your units hurry past the Ballista (and have one archer stand on that tile) and take control of the narrow bridge east of that Ballista.
You should also try to send units to those four houses to get prizes of 10000 gold, a Master Seal, a Dual Club, and a Dracoshield.
Be aware that when you use a Fire Orb or Ballista, you don't get to see the predicted damage that you will deal to the enemies. In other words, you will attack as soon as you have chosen where to target.
Also, keep in mind that the Fire Orb and Ballistas can't actually kill the enemies. They will bring the enemies' HP down to 1, but no lower.
Soon, some Sky Knights will show up, and they can fly over the water, so you won't be able to block their progress by standing in their way.
However, you will soon get some help from Camilla and her retainers Beruka and Selena. Camilla and Beruka can both fly, so they can help a lot against the increasing number of flying enemies that appear.
On the sixth turn of battle, Takumi will use the Dragon Vein to drain all the water from the port. This is a big problem, because now all of the walking enemies can simply walk across the dry land where the water used to be, making it much easier for them to go toward those four green tiles that you have to protect.
So after the water drains, you should shift your focus to killing the enemies that are closest to the green tiles. You can retreat to better defensive positions if you want to, but you might want to try a more offensive approach of just dealing as much damage to the nearest enemies as you can without letting your own units die.