First off, let me repeat what henning said: "They've seen your CV, and they invited you for a reason." So you evidently have enough to have gotten through the door.
As per Massimo, "the overwhelming majority of people do not have the best CV experienced people might have ever seen." Ya know, if they only accepted only the best CV's, there would be no need for interviews.
While my experience isn't exactly the same as your circumstances, I'll share anyway :->)
Back in the day when I applied to graduate schools, we knew in advance that most of the interviews were adversarial. Perhaps it was a misguided way to see if you could handle stress. Why that their approach, I don't know, especially since 30-60 minutes of uncomfortable cross examination could never duplicate the demands of the program or reflect one's ability to handle stress.
There's no way that you can prep for some specific unknown shot over the bow, only how you are going to manage it. But you can prep your mindset. Some things that I would suggest (some mentioned by others):
Make eye contact and shake hands when introduced.
Make eye contact and during responses.
Do not criticize, rationalize or make excuses.
Do not try to impress them with big words.
Respond directly and concisely. Stay on topic.
When you're done with your answer. Stop talking. If the interviewer remains silent, don't try to fill the gap with meandering word salad. If it's uncomfortable, confidently ask him/them a relevant question.
Try to respond conversationally. Don't drone on. Avoid poor speech etiquete (umms, errs, ahhs and long pauses).
Be affable, personable and professional.
Demonstrate interest and knowledge regarding whatever you will be involved in but do not try to impress/snow them.
Show some personality and confidence but no conceit, arrogance, superiority or anything negative.
I had one interview with a doctor and a professor who sat about 150 degrees apart with me in the middle on a swivel stool. They played off against each other, putting me in the middle. One was black and one was white. As an example, they raised a racially tinged issue and took opposing sides. They then asked for my opinion and the one I disagreed with went on the assault. At this point in my life, dealing with such an interview wouldn't bother me much since I have more knowledge, as well as the experience and perspective that decades of life have provided. But as a young kid whose future depended on admission, it was stressful. But nothing in my poker face or reaction reflected that. Stay calm and cool. You are accepted or you are rejected. All you can do is make your best effort and if this one doesn't work out, maybe the next one will. Good luck!