Because they had to pick out the crimson spheres at random and make sure they weren't being tracked, Rean's group took over twice as much time for each sphere. However, none of them cared about it. They had to keep themselves alive first before heading to the area Kentucky and Celis were keeping an eye on.
In any case, the method was effective. There were two instances where the demon beasts noticed their presence and tried to trap them. However, Frin howled from far away, using his Spiritual Energy to amplify his voice. With that, Roan and Duran immediately retreated before getting any closer to the crimson sphere. Rean and Frin, of course, left the area straight away as well. They didn't even try to come back to the sphere and instead chose another one.