About this (Kamihime x DP) crossover event

I've got everything:
Kamihime:
-SSR Weapon
-Event Eidolon
-Event Kamihime
-All Event SR weapons
Dragon Providence
-All crossover quests complete


Only one thing left to do and i have a question about this one particular crossover mission. How can i unlock Ragnarok difficulty in Kamihime? I've done countless battles (every difficulty, most of them were expert ones) and theory about chances is useless. How Ragnarok difficulty in events exactly works?