Eric thwarts the witches by accidentally hitting himself with the beam, which looks a lot like the ones from your local observatory's Pink Floyd laser show. The witches say the beam should have killed him, but he just shrugs it off. He then orders all of the witches to leave and they just ... do. Even though they could all go to jail for attempted laser murder, they just go home and watch The Craft again instead, because Eric said so.
I can't stress how normal this episode is, other than the Satan stuff. The B-plot is about another character being afraid to fly. Why didn't Eric die from being hit with the laser beam? In the Boy Meets World sequel show, Girl Meets World, we learn that he grows up to be a senator and not a demon fighter, which is, in my opinion, a huge missed opportunity for a spinoff. Boy Beats Underworld? Shut up, you know you'd watch it.