Ok so my opinion is that the guardian would win because Gurdians have actual powers because of the travelers light. And yes I know it you can say that master chief could just kill the ghost but here's the thing, if it's that simple then why didn't the fallen or any other enemy in the destiny universe do that in the first place. I think the spartan would have a very hard time trying to kill the ghost. One the other hand Spartans are super solders that have incredible strength but like I said before they have no true power. In the end a Gurdians could easily take down a normal spartan but master chief being the top spartan would be hard but would still lose in the end.