There wasn't a way that Clark could say that he knew what floor the base of the blood moon cutlists were on. Well, that was only partially true. Devi had been invisible this entire time. He had been prepared to fight if need be, but he had not wanted to be seen. A defending role so to speak.
"My partner just came by from the dungeon. He scouted down to the fifth floor while avoiding any beast energy he sensed. That's where the most danger is. I would say that's where the base is." Clark said this in a tone that left it up for debate.
This was common with people who did not have their beasts specifically for scouting. But those that had beasts for combat that could scout a little might get a few vague ideas. The elves believed Clark instantly. Mostly because they did not think that the humans were far enough along to scout properly in the first place.
Creation is hard, cheer me up!