It seems to me like one of the major obstacles to terraforming a planet is to give it a magnetosphere. In our solar system, this is accomplished naturally either by a hot iron core (for example, Earth), or by a heavy, thick atmosphere (for example, Venus). Neither of these things are easy to produce on a planet that doesn't already have them.
A workaround I've considered for this is to position an object in between the target planet and the star, to block out harmful rays while letting through the necessary light/heat to support human life. I want to think this would be as simple as fitting a large craft with a series of mirrors, and positing it to 'follow' the target planet as it goes around the star. But since I haven't heard about this kind of idea I'm guessing there's something I'm missing.
So, would this idea work? Or if not, what would go wrong?