The thing to keep in mind with most Case Studies is that there isn't one right answer. Most companies want to see how you would solve a problem and how you would think through implementing a solution. They know that you aren't going to have all the answers and know exactly what to do, because they know you are still a college student. The majority of companies are hiring you for your potential, not necessarily for who you are right now. They are looking for someone who can present themselves well, be articulate, be respectful, and is "coachable."
In your preparation, I would find a couple of case studies online and talk through them with someone you see as having some "Business knowledge" or Problem Solving prowess. This could be a friend who is a Business major or and Engineering major (some of the best consultants are engineering majors). Practice presenting and defending your answers. Have your buddy try and challenge you if you don't give adequate supporting information. Have them ask challenging questions to try and catch you off guard.
My last advice for you is to not try and pretend you know more than you do. If your interviewer asks you a question you don't know it is acceptable to say that you wouldn't know what to do in the situation. It is ok to say that you think you would do X but that you would check with your team first. I have never been a fan of faking it till you make it, because chances are your interviewer will see you as inauthentic and fake. It is ok to make some assumptions, but make sure you note them. Most case studies are purposefully designed to be vague, so you will probably have to make some assumptions. As I said before, your interviewer is not necessarily expecting you to come to a single correct answer, they want to see how you think.