Hard to see a fighter who could outbox him over 12. If you want to go historical then over 15 rounds it seems impossible since stamina is his biggest strength. He has every trick in the book. I don't think he can be outboxed. I think you have to bring the fight to him. I think a prime and motivated Tyson or Frazier could give him serious problems and potentially stop him. Styles makes fights. If you sit back and box with this guy he will take you to school.
It's no secret the guy who gave Usyk his toughest fight wasn't a giant it was a Cruiserweight in Breidis. These giant Heavyweights now haven't got the workrate or cardio levels to live with Usyk in the later stages of a fight. Look at Bowe vs Holyfield, Holmes vs Norton, Ali vs Frazier, they all went at it like hammer and tongs consistently throughout the entire fight and that's the kind of stamina you need vs Usyk.
Vitali's an interesting fight, tall, powerful, can do rope-a-dope without a rope, braver than Wlad at taking chances, better chin, had a natural feel to the game, also good stamina. But Usyk has better output and speed, very hard fight for him tho.