In a world of beasts, the Grogs were one of the very few beasts with unknown or changing characteristics. In the sense of the human world, the Grogs acted in similarity to the atmosphere of their surroundings just as cold blooded animals are able to adapt to the various temperature of their environment.
Changing their location to the depths of a thick forest that gave way for bright and hot rays of the sun to peep through, gave them an additional strength due to the vibrant and energetic atmosphere.
On a normal basis, supreme beasts were weaker than Divine beasts and so we're Divine beasts weaker than celestial beasts. It was even worse when they were like 9 grades apart.
Bargaining with the Grogs for the core they were after would most definitely and without a doubt yield no result. Stealing it without raising any alarm was the least they could resort to but it appeared that over time the Grogs became aware of the happenings.
