"Trump can begin by complimenting Carson’s “inspirational” story and medical brilliance, but then pivot to his lack of leadership experience. “He’s a wonderful guy, but we’re not fixing brains here, we’re getting killed by China. We’re not separating Siamese twins, we’re separating Mexico from Texas with a beautiful wall!”
The above is the way to do it, and it's basically true. Carson is a nice guy, but not qualified to be POTUS. Trump isn't either, and has a temperament and judgment problem to boot, but a big chunk of the Pubs don't give a damn about that apparently. It's sad the two current front runners for the Pubs are so totally unqualified to be POTUS. It makes Mittens look like a veritable political God in comparison. Where have all the flowers gone? But miracles can and do happen, and maybe Kasich will rise and save the polity from itself. Keep hope alive! 
The idea of having someone you identify with and understands "people like me" rather than an expert is a large part of the appeal to both Trump and Carson. So this is one place Trump can't go. I don't think Trump can do much to attack Carson without it backfiring.
The only way of attacking Carson that wouldn't backfire is to claim that he is low energy.
That's something Trump could use occasionally, but I doubt he'll use it systematically.
