After getting past Xander, you must defeat King Garon to end his evil rule.
Before the Battle
This is your last chance to go back to your castle. Once you complete the chapter 27 battle, you will be in the endgame and will not be able to save or return to your castle. Make sure to prepare before starting the chapter 27 battle.
Defeat the Boss
You only need to defeat Garon to end this battle.
From where you start the battle, there is a General straight ahead, and a Berserker and a Hero in the rooms to the left and right. The General is particularly weak to the Hammer weapon, so use that against him if you have it.
The Berserker, equipped with an axe, so he is weak to swords and tomes. He has lower RES than DEF, so tomes are your best bet. The Hero uses a sword, so he is weak to spears and hidden weapons.
Farther north, there are a couple of Great Knights. Since they are mounted units, they will be weaker to Beastkiller weapons. They carry both a sword and a spear.
To the left and right of those Great Knights, there are a couple of Bow Knights. They are also mounted units, but they wield bows.
There is a Berserker in the center of the map, and he has the skills Vengeance, Counter, and Swordbreaker. You won't be able to attack him with swords, and if you attack him from an adjacent tile, he will deal damage to you at the same time. Therefore, use ranged attacks against him.
To the northwest and northeast of this, there are four Wyvern Lords. They wield both an axe and a spear.
In the northwest room, there are three Malig Knights that wield tomes. Also in the northwest room is a Strategiest, who wields a tome, and also has the Fortify rod, which will restore HP to all enemy units in its range.
In the northeast room, there are three Dark Knights. They wield tomes and swords. Also in the northeast room, there is a Maid who wields a Silver Dagger, the Recovery rod, and the Fortify rod.
Three Generals block the path to Garon. Then in the northernmost part of the room, there is a Strategist, a Sorcerer, and Garon himself. Deal with the Strategist and Sorcerer first, then have all of your units attack Garon until he dies.