@Manghuwa
You're not wrong about MC being overconfident, though he has a reason for it. It's been over a dozen floors since he's had to "go all out" and feel the sense of mortal danger. Even in the floors where he needed multiple rounds/resets to beat, he wasn't really in mortal danger in them. Not like he was in the first handful like when he fought Itty.
He's wanting to see what he's capable of which is why he's even wanting to fight the demon king without the sword. He wants to see what his limits are so he can improve on them and get stronger.