Fire Emblem: The Sacred Stones Tips

Kill Demon King in 2-3 Turns
First give Erika Sieglinde and Audhulma, and Ephraim Siegmund and Vidofnir. It's helpful for them to be between levels 14-20, so train accordingly.

Next start the final mission. Make sure you have a healer with you, and that you place him out of range from the demon king. Then all you need to do is attack with Erika and Ephraim and heal with the healer.