Oh certainly Scott can hurt these guys. But I dont buy a one hit situation.
I believe that the Skullcrawler's feat of ignoring the boulder to his head is a better durability feat then the Leviathans building ramming feat. And really, it was effectively ignored. It did flat zero to the Skullcrawler. Which sure, the buildings don't hurt the Leviathans but buildings are mostly hollow structures.
To lay out my stance in full: I believe Scott is physically superior to either monster. Bigger. Stronger. I believe he can badly hurt them with any solid blow he lands.
I don't believe they are one shot from said blows. Just solidly damaged and certainly cant take very many.
I believe they are faster and much more agile then Scott.