First, a warning, there will be three units which attack your ship from the side opposite of the ship that is attacking you after L'Arachel's ship arrives, don't leave your healers unprotected in the back, as they can't defend themselves! Also, give someone a torch or the staff torch, it's foggy in this level!
For this battle I would suggest using your stronger characters as there isn't a lot of cover for you to level the weaker ones. If you upgraded Ross to a Pirate you can send him out on the water, but he's more use next to Garcia, uping his fighting abilities. Keep your healers near the mast pole, if you only selected one or only have one remaining, put them between and then have two upgraded characters on either open side, in this way you can effectively protect the healer, add characters to protect the healers if you have two of them with you. L'Arachel may die quickly, so you need to go and recruit her quickly. Have Ephraim be 'rescued' by one of your flying characters (Pegasus rider or Wyrvern rider), and carry him over to the ship. When you are close enough that Ephraim can get over to L'Arachel in one turn, drop him. ON the next turn send him over to L'Arachel to recruit her, then recruit Dolza with her. Protect L'Arachel and kill the remaining monsters.