And why was Frieren hovering above, smirking and watching like an asshole while her allies and pupil were in mortal danger?

Her OP-ness would have made easy work of those two demons, and then where would the drama come from?

Her experience told her that Fern and Methode were more than capable of handling that, kind of in the same way they left Stark behind with much of a second thought. Frieren rarely shows her power, pretty much the only time she had to was when fighting against her mirror image.

Also she probably had the ability to watch through the fog an intervene at any point if thing went wrong, she’s just that OP.