“The group departs for Nohrian-occupied Cheve, seeking Ryoma. They secure a ferry to carry them across the sea but encounter a familiar, unexpected enemy.”
Frozen Sea (凍てつく海 Itetsuku umi) is Chapter 12 of Fire Emblem Fates in the Revelation Version.
The script for this chapter can be found here.
The Avatar and their allies are sailing across the sea, on their way to find Ryoma. The boat they're on comes to a stop only to find the water completely frozen over with Camilla and Flora on a nearby enemy ship. Camilla states Garon has ordered Camilla to kill them. Your maid Flora, is also being threatened by Garon to kill them or else the entire Ice Tribe will suffer.
Enemy positioning is the same on all difficulties.
The map consists of six boats on the frozen sea, where the middle four have three starting positions each. Enemies will traverse the Icy Sea tiles (which halves the movement of any non-flying unit) to attack the player on their boats. While the enemy can, the player cannot travers the Icy Sea tiles (with the exception of flying units), and will be limited to their starting boat for the start of the map. On the sixth turn, Dragon Veins will appear to create paths for the player to move from boat to boat.
The top right boat with Camilla and Flora (the boss) and the bottom left boat with Selena and Beruka are both longer and start out with enemies, while spawning more on later turns from their stairs. The bottom left boat with the retainers contains unpromoted Fighters (and a single Berserker), Outlaws, and Dark Mages, while the top right boat mostly contains their promoted forms, Berserkers and Sorcerers (on all difficulties).
This map can be challenging due to the forced splitting of the players units. In case healers can't be spread out in an even enough formation, every unit should carry at least one stack of Vulneraries or Concoctions. The two leftmost boats will most likely see the most action due to enemy placement, this unfortunately being where the Avatar is forced to deploy, so at least one healer, preferably Felicia/Jakob, should remain nearby to support them as needed.
It is possible to group all of your units onto one boat through the use of Pair Ups and three flying units (Subaki, Reina, and one other unit who has been reclassed), which will force the enemy to trickle to the chosen boat. While it does mean that all of your fighters, healers, and Azura will be together for the wait until the Dragon Veins pop up, care should be taken to avoid the Outlaws, as they can snipe at your flying units while ferrying units to the chosen boat. Also be wary as Camilla can intercept your flying units before they can reach a boat. Otherwise, simply pairing up everyone before the chapter begins can leave you with two well-armed boats to manage instead of four shakily-armed boats.
You will need to keep an eye on your softer units no matter what you do, as the size of the boats you are on will more than likely mean any enemy unit on board can run up to them and attack, easily swarming them if you can't take out the enemy in time. Using long-ranged staves and wands is recommended if you can get any, that way your healers can stay away from the action and continue their healing without running the risk of danger.
Kagero, received in the last chapter, can be very useful in defeating Flora, as not only can Kagero debuff Flora and weaken her further with Posion Strike, but her personal ability (returns half the damage and debuffs dealt by enemy knives/shuriken) will trigger on hits. If this does not outright kill Flora, it will at the very least leave her very weakened for any of your other units to kill, giving them a great amount of EXP if needed.