iwand magic vs. card shadow magic
wizard duel vs. card duel
He-Who-Must-Not-Be-Named against Marik's dark half
no summoning Ra for Marik
anything else goes
Shadow Games have a usually fair playing ground making it an even chance for either Voldy or Marik to win. However, the usual price to pay is your soul. Considering the whole Horcrux schtick Riddle will purposefully lose, get up, then promptly kill Marik.
Voldemort is one of the most powerful Occlumens and Leglimens in the planet, he isn't some stupid moron who can be easily possessed. And for cereal, Marik will be Avada Kedavra'd by then.