We will have to wait and see where the story takes Cortana, but she is a part of Chief. They are together even when they are apart or "against" each other. Cortana did entrap Chief, but only to save him from her plan to activate the Halo rings. You can't talk about one without the other - even when theoretically pitting her against Ultron.
If Ultron could manage to kill Hulk, thor, and Iron Man I think he could easily crush Chief. Especially since Ultron's chassis is made from indestructible Adamantium. And like I said before, Cortana doesn't pose a threat to ultron.
Cortana controls the Guardians, has swayed the AI of several species, has an endless stream of Wardens, directs the Promethians, and can destroy all sentient life with the Halos.
Ultron couldn't even kill JARVIS right.
So lets hypothesize -
The real threat to Ultron isn't Cortana; it's Chief. Chief is built with technology that is 500 years more advanced than that of the comics... We don't even know where Adamantium stands in this future. It also seems that everyone has forgotten what the Librarian told John when they met in Halo 4. Remember that intangible characteristic of John's that Dr. Halsey and Cortana have always referred to as "luck"? Turns out that John has been watched over and protected by the imprint of the Librarian Forerunner because he isn't just any human, he is THE Reclaimer. He is "the culmination of a thousand lifetimes of planning". Nothing about John is by "accident" - He is the eventuality of the physical evolution the Spartan Project, Mjolnir armor, Cortana (and her advanced abilities)... all of it was to ensure that he would be the one to reclaim the universal Mantle of responsibility. The Librarian has imprinted John with ancient seeds of the past to guide his evolution to his current state. After John meets the Librarian, she "accelerates his evolutionary journey" - we don't even fully comprehend what the consequences of this are or what he might be capable of yet, besides that Chief's human-genome cannot be deconstructed by the Composer. Not only was he raised in battle, he's been enhanced by science and technology far beyond that of Iron Man, and he holds a supernatural connection to the fate of the universe.
He cannot lose - not even to Ultron.
How many times has Ultron been defeated already? He has to keep coming back as different iterations because he's lost so many times already. Chief hasn't lost. Ever.
Actually.. the latest iteration of Ultron is merged with Pym and is a human/machine hybrid.. much like Chief. Ultron needs Pym like Chief needs Cortana. Not only is Chief imbedded with technology to make him into a super soldier, but also having his DNA altered to a point that it cannot be trans-mutated by the Composer. Lets assume this DNA evolution that the Librarian enhanced Chief with also makes him immune to the nanite virus that Ultron uses to infect organic creatures into drones. Also, with Cortana by Chief's side - he also has an unlimited army of drones (Prometheans and AI alike). And with Cortana now in control of all Forerunner artifacts she has entire star-systems of shield worlds, planets, even-further advanced technology at her disposal.
According to Wiki:
and yeah, yeah... Adamantium. It's not Ultron's outside that you need to worry about; it's his inside. Ultron's internal systems have been damaged multiple by times before regardless of the adamantium armor - namely by Johnny Storm, the Scarlet Witch, and Wonder Man.
- Ultron has superhuman levels of strength, speed, stamina, durability, and reflexes.. Chief has this imbedded in his body, but also with the assistance of Mjolnir armor
- Flight at subsonic speeds, Ultron has Chief on this one without the use of a ship. But Chief can fly pretty good, for a brick. (I miss Johnson*)
- Concussive blasts of energy.. Chief has weapons that do the same thing.
- Encephalo-ray.. not even sure this would affect Chief in the slightest with the Librarians DNA evolution. (ditto with mind-control and hypnotic commands)
- Ultron's genius intellect.. this is why Chief has Cortana
- Creative intelligence, superhuman analytical capabilities, ability to process information and calculations with superhuman speed and accuracy.. Also Cortana.
- Self-repair.. Mjolnir armor
- Program transmitter that can beam part or all of Ultron's memory/personality system into other computer systems or duplicate robotic bodies.. Cortana can do this too.
- Can control other machines remotely.. Also Cortana
- Ability to restructure its physical form.. think Cortana using hardlight to create a physical body of herself. She is no longer a hologram.
I think the odds are pretty even. It would be one hell of a shoot out.
I get it, this is a hypothetical argument and we're both cheering for who we want or believe would win in a battle for the galaxy. This debate could go on forever and neither of us would change our minds - but seriously... You are not giving Chief nearly enough credit or respect for his actual powers. We just tend to see him as a "stronger-than-normal" marine or ODST. That's not even a microcosm of the truth. There is more to him than we can even imagine and the story has just begun to scratch the surface.
You not only underestimate Chief, but you dismiss Chief way too ignorantly with your matter-of-fact statements.
Just give respect where respect is due.
Yes in every single way. After looking at Wikipedia, I see these truths. As well, for I read the official Halo books as well as play the games, I know that the Spartans accomplish the impossible. John as well as the other Spartans accomplish what cannot be accomplished time and time again. Even facing Halo 6, I and probably you guys see absolutely no way Cortana can be defeated, but I know 343i would not end Halo 6 without John winning, and defeating the impossible. You say he cannot defeat Ultron; I say the Chief could do the impossible like the Spartans always have.