Doomsday has no notable superspeed whatsoever, and the only reason he tagged Superman or WW is because of PIS. This is borne by the complete lack of "blur" style speed shown in his fight, wherein Doomsday is only ever seen moving in normal speed, and his consistent inability to tag Batman. Based on this, Diana should be able to one-shot Doomsday in the arena.
Thanos is able to fight Thor, Iron Man and Captain Thormerica at the same time and was winning. You can not say that he "is not known for agility". Doomsday is also a ravening beast with no skill or fighting ability whatsoever, something Thanos has in spades. Based on Doomsday's feats in his one appearance, I can't recall any strength feat that eclipses Hulk, or Thanos for that matter, who is arguably stronger than the Hulk.Also worth noting is that Doomsday is considerably taller than Thanos, and Thanos isn't known for his agility. He's gonna have to cut a considerably faster and stronger opponent with regen down long enough to get a shot at the head. And even assuming a clean decapitation would kill Doomsday, Doomsday's feats at least suggest the ability to regenerate from partial decapitations or having a blade simply embedded in his skull.