Ironmam wouldn't even be able to defeat Thanos ship, let alone Thanos.
I don't really see Issue with Tony beating the Black Order, if Maw is killed right away with a quickdraw beam to the head or something. That is to say, I definitely thnk Tony could beat Proximia, Corvus, and Cull at once if he's got bleesing edge, hulkbuster, and iron legion.