There was a reason that Rean thought he would find the right moment, the formations prepared for the demon beasts. As mentioned before, the Soul Rulers organization had prepared formations to help deal with Phex's group, but they haven't been used yet. That being said, they had been deactivated since formations capable of holding Stage Nine Demon Beasts just consumed way too much energy. They would only be turned on when Fikin's group needed it. Obviously, that would be at the moment they gave up fighting and decided to run away.
Of course, there was no guarantee that it would work, but Rean thought it to be the best chance. Sister Orb kept an eye on those deactivated formations during the entire extent of the battle, and she just noticed those formations starting to activate. Since that was the case, Rean immediately used his connection with his Light Element to release Roan's Dark Element into the bodies of the Divine Demon Beasts.