I'm not a teacher. That said, I would just like to add to the excellent definitions given above that "formidably" in this context often carries a connotation of fear, possibly discomfort, or grudging respect. The phrase "formidably intelligent" or "a formidable intellect" is often used here in NZ in a political context, when politicians are discussing an opponent. It is a compliment, but it definitely suggests that the person using the description would be happier if person being described was not so "formidably" intelligent.