the Demon King is really hard. Unlike the Dragon in Fire Emblem Rekka no Ken, The Demon King moves. You battle the Demon King after you kill Prince Lyon who is very strong, an S in Dark Magic, and is a Necromancer (same thing as a Dark Druid in previous Fire Emblem), has a Naglfar (S Dark Magic), and has 73 HP. Ok, back to the Demon King. Hopefully your Mrryh still has atleast 2 uses of Dragonstone left. She does 45 damage max. It works best if your Mrryh is a level 20 Manakete with 20 Strength, Defense, and Resistence. The Demon King might do a double strike and if your Resistence isn't high enough, he might do a critical hit. After your Mrryh attacks, use Seth (hopefully an S in lances) with Vidnofir, Eirika with Sieglinde or Audhulma, possibly Saleh with Excalibur, Garcia or Dozla with Garm, and Ephraim with Siegmund. Innes or Knoll with Nidhogg and Gleipnir might not work except for a kill. They have low HP unless you train them a lot. Now that you've killed the Demon King, head over to Tower of Valni or the Lagdou Ruins to get some pretty good characters like Lyon, or Valter. You can also get some royalty like King Fado, King Hayden, and Queen Ismaire.