The constitution of those monsters was far greater than that of common human cultivators.
The palm attack that Lu Xuan had unleashed was called the "Crumbling Mountain," a move capable of devastating force within an instant. When honed to the extreme, the force would be on par with its namesake—able to bring down a mountain with only a single strike.
Even though the power of that attack at the moment couldn't compare to how strong it had been when Lu Xuan was in his pinnacle state, he was nonetheless easily able to clap down a Deity-level fighter with it, hard enough that the victim would have a hard time getting up.
Yet the rat king only rolled around for a moment before getting up again.