In this chapter, Elise will join your army along with her retainers, Arthur and Effie before the battle starts. Charlotte and Benny will also appear as enemies in this map but you can recruit them by having Elise talk to them. There are no turn restrictions in this battle and you have to route the enemy to complete it. There are three contraptions you can use here; two ballistae, and a fire orb. Use this to soften up the enemy forces near your location.
There's a lone dragon vein to the SW, near Charlotte's ship. When activated this will lower the tide, making the river passable. There are also 4 houses you can visit.
House items:
NE: Wane Festal
SE: Master Seal
NW: Physic
SW: Levin Sword
For the strategy, you need to set your most powerful and sturdy units so you can visit the villages outside your base. Effie, your avatar, Camilla, Gunther, and Reina (or whoever character you trained) will be your vanguard. Leave a handful of defenders at the base, preferably those who can operate the turrets. You can leave Azura at the base so you can use her Sing command to fire another shot using a turret.
The some portions of the partitions in your base can be broken so take note of their positions and prepare to send one of your melee units to deal with the attackers in case they break through. Surprisingly, enemies will prioritize taking down the partitions first. Ranged attacks behind these partitions are quite effective.
The enemy forces to the east will be led by Benny while the enemies on the west will be led by Charlotte. If you want to recruit them both, you have to take care of Elise and make sure she'll be able to talk to them before they get defeated. To reach Charlotte and Benny easily, you'll have to partner her up with a flier, and maybe bring Reina or another powerful flier escort to deal with the enemies. Fortunately, the two of them won't leave their positions and attack your units unless you get near them.
Before starting the battle, position the turret operators near their respective posts. Keep a backup healer ready and designate the forces that will guard both sides of the base. The western side has a full blockade with only the partition in the middle that can be destroyed. Placing a tank (like Gunther or Effie) with support and healer behind it will effectively block the enemies there. To the east, you'll be dealing with knights and cavaliers so powerful mages or fliers can be quite effective in defending that location. You can then amass your vanguard units in the frontline along with Azura who will sing to the operators and allow them to fire again. Leaving a healer near Azura's range will also allow her to heal more units especially those tanking the frontlines.
When positioning your characters, always have the enemy attack overlay activated so you can see their attack range. Don't spread your forces aggressively; just let the enemy come and thin out their numbers before moving out. Be careful of the enemy paladins as well. They have high defense, high attack, and ballistae doesn't seem to damage/hit them that much.
Once you've whittled down the enemy forces near your base, you can now head out and start aggroing the other enemies especially those near Benny and Charlotte so you can deliver Elise to them. After talking to them, they'll immediately become an allied unit that you can control. A handful of reinforcements will arrive but not that much to threaten your progress. However, even if things seem to be under your control, leave some forces in your base so you can react in case enemies start to flank you.
Once you've successfully recruited Benny and Charlotte, you can now continue with the battle and make your assault on Silas' ship. After defeating him and all the remaining enemies, the mission will be complete. Silas will join your party as well.