I have completed all the missions and now I am at the last mission. It seems impossible to beat the demon army with Kujin, but you cannot stop the hero fromm attacking to the castle do you know is there another way or should I restart the mission and come there a few weeks earlier?

You have to Use Kujin.. also i don't think the armies in the castles are growing, so while Kujin is most certainly one of the toughest battles. you should be able to do it if you play it right.

The armies in the castles aren't growing and you have to use Kujin but you dont have to beat the enemy with her. I beat him with Gotai