If you truly believe your opponent cheated, the proper way to report it is here:
Since Battlegrounds is in early Beta still, bugs can certainly happen. It may help if you post the link to your screenshot (be sure to surround the link with a single backwards quote, as you get from clicking the Preformatted Text </> button, `Like this`).
One guess: I wonder what the limitations of Shifter Zerus are. Though that is a 3 star minion, you would not get to buy it on your first turn. Maybe Infinite Toki’s power can somehow get you a 3 star minion (Zerus) very early and maybe you can get (very) lucky and get a 6 star minion from it?