After meeting the Rainbow Sage at Notre Sagesse, you go to Fort Macarath to rest.
Defeat the Boss or Have Corrin Escape Within 16 Turns
You will not be able to use Elise in this battle.
Around the battlefield, there are medicine pots and poison pots. If a medicine pot breaks, all units within two squares of it will either be healed for 30% of lost HP, will get +4 to one stat until the end of the battle, or will get +2 to two stats until the end of the battle.
If a poison pot breaks, all units within two squares of it will either be damaged for 30% of HP, will get -5 to two stats (it will recover each turn), will have Move stat set to 0 and Avo stat set to -20 for one turn, will become unable to use magic, rods, or staves for one turn, or will have max HP halved until the end of the battle.
Put the cursor over a pot to find out if it contains medicine or poison.
You can use the medicine pots strategically to boost your team's stats, but unless you have a way to attack outside of the range of the poison pots' effects, your ability to use poison pots against the enemy is limited. You might want to just stay out of the range of the poison pots' effects.
Near the center of the map, there is a Dragon Vein that you can use to break all of the pots in the entire map.
After a couple of turns, reinforcements with bows will arrive in the southwest and southeast corners of the map.
Your objective is to either defeat Ryoma at the top middle of the map, or have Corrin escape within 16 turns. The escape tiles are behind Ryoma. You will lose the battle if you do not complete it within 16 turns.
There are two chests, in the northwest and northeast of the map. The one on the left contains the weapon Armorslayer, and the one on the right contains 5000 G.
A couple of enemy units are carrying Door Keys, which allow you to open the doors in the northwest and northeast corners of the map. This allows a bit of a safer approach to those chests.
One of the enemy units is Saizo, who is carrying a Flame Shuriken that he will drop when defeated. Ryoma himself is carrying a Secret Book. And another enemy is carrying a Kodachi.
There are several Shrine Maidens in the northern half of the map. Some of them can heal the enemy, while others can inflict negative status effects on your units. One in the northwest corner can cast Freeze, which will immobilize one of your units. Be careful not to let your units walk into her Staff range or they will be stuck there for a while. Kill that Shrine Maiden when you can.