The aesthetic changed from Dragon Warrior to Soul Eater at the drop of a hat.
That's your cue that this isn't a dungeon that you were prepared for.
It'll be interesting to see how this plays out. I seriously doubt the kid will kill them, but they're going to get their asses beat. I'm pretty sure the entire hero party working together couldn't beat Grandpa Thundergod, and we don't even know what his other spirits are or do. I'm apparently not the only one hoping that the ninja gets a job offer.
My prediction is they get stomped with little effort because they're cocky, and the kid lets them run away because he's not the kind of person to just kill them at the drop of a hat. The hero and his hoes will run back to civilization and spread the story of this new Lieutenant of the Demon Lord that's crazy strong, because if they don't play up his strength they'd just be admitting that they're incompetent. And that's how the kid will end up with comedically absurd infamy. The ninja will probably split off of the group, because she'll know the truth, which is that the hero is a moron, and she doesn't want to babysit his party any longer. I'm not sure if she'll join the MC sooner or later, but she will probably join him eventually.