The first map your on in this chapter
The second map your on in this chapter
Final Chapter: Sacred Stone is Chapter 21 of Fire Emblem: The Sacred Stones, and the last chapter besides Creature Campaign. It takes place inside of the Black Temple. The boss is Lyon, who commands his monsters to stop the royal twins from reaching him. The objective is to kill Lyon. After said objective is completed, along with a short cutscene, the player must face the Demon King and the monsters that he summons.
Strategy
Seperate your team into two groups, one to go down each pathway. Don't forget the chests. Conserve your holy weapons for the Draco Zombies ahead. send your units with higher resistance up the stairs, as the gorgons can and will use Shadowshot. After taking over the stairways, catch Lyon and remaining gorgons in a pincir movement. Use high-power units like Amelia or Myrrh to take him down. For the Demon King, try to use a high movement character like Cormag (preferably with Vidofnir) to deal quick damage. Make sure that someone has Latona and can use it. Move everyone closer and end the turn. Deal with monsters in the way and use Latona. Repeat this until the Demon King falls dead.
Another highly viable option would be to 20/20 Joshua out, give him Audhulma, and let him kill Formortiis. Since he is a swordsmaster, there is a good chance he can easily kill the Demon King in a matter of 2 turns.