If words failed about twice, I would've convinced him using sheer force. If you beat him down until he can't do anything anymore, and then properly explain, and then heal and free them afterwards, they'd at least know you could've killed them but clearly didn't. That'd go against the idea that Naofumi killed the other heroes, making it more likely for anyone, including the Spear dood, to believe him.

If they still didn't after that, then the heroes will get killed by gigabird.