Chaos Grundy is powerful but a liability in this fight, Onslaught can fairly easy remove him leaving himself and Thanos against JLU Amazo.
JLU Amazo has shown that there apparently is no ending for his ability to copy his foes powers, Thanos he can copy the question remains if he can copy Onslaught..
However even if he cannot I believe that JLU Amazo based on what he was capable of accomplishe when he returned will win.
I agree, the guy was about to destroy the universe just by thinking about it. He takes this 10/10 by himself and quite easily (even though onslaught is a beast and there is next to none that can beat him.)
It wasn't that, that puzzles me, it's that Onslaught is made from either two ore four different beings (depending on version). If the powers steam directly from him ore he simply taps into those he has absorbed when he requires their energy in from my point of view interesting. If the last is the case then Amazo will see the same in Onslaught as he saw in Batman ... nothing.
But the ring is a object, the persons that Onslaught absorbed was located inside of him in something that at times almost seemed liken a alternate dimension.
I'm not aware of which version the Threadstarter intended to use.
But if we take the first Version with Xaviar inside of him then X-ray would most likely work, I cannot remember the exact circumstances surrounding Xaviars capture so until I have checked it, I will say that against that Version he can possibly copy his powers.
However with the Second Version X-ray isn't from my point of view going to help since again it appeared that they where located in some sort of alternate dimension within Onslaught, but again that depends on in which way Onslaught accessed the powers from his prisoners. Was they a part of him ore did he draw them when needed...