In episode 07 of Nisemonogatari, we see that Karen gets into a physical fight with Araragi and causes a lot of destruction which is unlike a nomral human.
You have to keep in mind that the story is never ever told to us by an omnipotent narrator. Depending on the chapter (or episodes/arcs in the animated series), a different character tells us his story. Koyomi Araragi is the character that is narrating the story most of the time, so what you read/see/hear will almost always be from his point of view.
To give you an example of that, look at how Koyomi lets us see Kanbaru Suruga's room: full of books to the point where it looks more like a pool of books than anything else. Of course, Kanbaru's room isn't really that abnormal, she's just a girl with a lot of books in her room, probably just piled up in a corner next to her bed and desk. But from Koyomi's point of view, these piles of books in the corner are so representative of Kanbaru's room that the way he'd tell us how her room looks like is "It's a room full of books".
The same principle applies to Karen: she isn't really that powerful, but she's probably the strongest person Koyomi has ever seen. Add to that the fact that he has been bullied multiple times by his sisters and you get the image of a super-strong, destructive being.
Don't believe everything you see in these series. Keep in mind that any character can lie to you and twist the story at will.