Beatin demon king

The point is beating it as fast as you can, reach it with Ephraim and take out some of it's life them use psychic to heal Ephraim, end the turn and it will attack or summon enemy allies. Forget the enemy allies and follow the same thing with Ephraim, if the enemy is still alive use your other's strongs units with legendary weapons to kill it. Good Luck!

The demon king is easier than fighting lyon,i beat him usign ONLY a lvl 20 hero gerik with audhulma and natasha healing him using physic.if you want an easy victory try using your strongest char and giving him/her a legendary item,for example lets say a lvl 15 swordsmaster marisa with audhulma could wipe him out in maybe two or three turns alone(with healing).i found another easy way of killing him and it starts for chapter 5 in serafew(this could take a few restarts)make sure seth is in the party.clear everyone out(and recruit joshua using natasha)and send seth to the arena to fight a little and gain soem moeny
Get natasha to buy soem heal staves and make her heal him whenever hes injured.over tiem with a little luck youll get him to lvl 20 with the sword or lance in s rank(depends on which one had higher skill.ive noticed the weapon with higher skill in it is used instead of choosing)whichever weapon was in s rank try the appropriate legndary weapon(lance vidofnir and sword audhulma)and send him to kill the demon king.within a few turns he'll have won the match without a problem.(also you may want to brign a few others for backup when he summons three draco zombies and a few other units)

I found the simple way to kill the Damon King was to just go up to him with Ephraim and his "Siegmund" lance (All my units were level 20 by the way) and then go in with Erika and her "Sieglinde" sword. I never needed to heal and it was very quick.

I used Seth Level 20 with Audulma, Dusseul level 17 with Garm, Moulder level 5 bishop with Letona and Rescue, and Myrrh level 20. Ross died in that battle but he was a beserker without any weapon so...tough luck ^.^ But the Demon King is pretty easy. WAAAAY easier than Lyon actually.