After defeating Iago and Hans, you show your Nohrian siblings the truth about King Garon.
Preparations Before the Battle
In the rooms on the left and right, there are max-level enemy units of various types. When choosing units to put into the battle, make sure to choose ones who have a weapon advantage over each of these enemies, and also make sure to pair somebody up with each of your units in these side rooms.
Defeat the Boss
The Shrine Maidens are all pretty easy to kill, and they can cause a lot of problems if you ignore them, so on your first turn you might want to kill all the Shrine Maidens.
Meanwhile, the units that you left in the south part of the map should go north.
After this, the units in the side rooms should try to kill the enemies in their respective rooms.
The Lancers and Merchant in the center will soon be in range of your units in the middle of the room. The Lancers have the Counter and Countermagic skill, which damages you if you attack them.
Once your side units kill the enemies in each room, the door to the stairs in that room will open, allowing your units to quickly reach the upper part of the map.
After you have dealt with all of the enemies in the center, you should head north to deal with Garon.
You should do a Battle Save around this time, because you will not be able to return to your castle after this battle.
Gather all of your units outside of Garon's range, then send in whichever units can damage Garon without being killed by Garon's counterattacks.