The Dungeon Boss gained power it had never seen before upon unearthing Leviah's spear, and the Demon used its body as a temporary vessel and enacted revenge upon the adventurers on its behalf.
However, the Dungeon Boss' power didn't come out from nowhere.
While the monster believed it had gained her power, Leviah had actually used Demonic Possession on the monster, and she forced it to burn up its soul in exchange for power.
Power that ran out at the same time the monster's soul did.
Leviah used her weapon to kill the adventurers, and she chose to absorb experience from half of them while taking skills from the other half.
Considering that they were able to reach the bottom of this dungeon, they were certainly very high level, and that gave Leviah a LOT of exp.
The Kind that took her from the Gold Level she had been weakened to, straight to the peak of the Master Level in on go.