It wasn't that the level 30 source beast had high potential, but that the treasure was located at the heart of the level 30 source beast. As long as the heart was eaten, Big Mouth's potential would increase.
This treasure was even higher than the treasure on the first floor.
However, a level 30 source beast, Lugh shouldn't be able to defeat it. There was a level difference of 7 levels. But even though Lugh couldn't defeat it, in this palace, there was a Sword of Light. That weapon could counter the level 30 source beast.
In fact, if no one knew that a level 30 source beast was the greatest treasure.
They would think that the Sword of Light was the greatest treasure here.
The Sword of Light was guarded by that level 30 source beast. When people obtained the Sword of Light, they would not provoke that level 30 source beast.