Hate filter makes everything look like nonsense, huh? But then again, I've been there myself with other series, so I shouldn't blame the haters too much.
Nothing in these chapters seemed especially outlandish to me. Frey already noted Isabelle is highly talented. She was just lacking good training methods because the word has forgotten them. But even so, 3 star apparently is pretty good among her peers. She's also doing nothing here on her own since apparently see can see exactly what Frey is doing with some mana vision. I imagine if you just read about this from some moldy scroll, it might easily cause a person to self-destruct. There are a whole lot of things in the world that are much easier if you have got a person show you exactly how it's done and you can just try to replicate the actions. That might be the crucial difference making her only somewhat damage herself, not explode internally.
I'd like to think that when Frey told her not to try to copy the method, in fact he meant that if she has got the determination, she should go forward with it. If she didn't, he would judge her not worthy. Nobody's going to stand at the top by only following orders. Frey/Lucas himself wants to defeat the tyrant overseeing the world, the demigod. We have already seen he didn't get where he did in his previous life by only following orders.