Alright, I'll give my first argument then.
One thing to note about Spartan Team Red, is that they were wearing Mjolnir Mk4 armor. Meanign physically they're likely weaker than John in his Mk5 armor we see in CE. Hell, they don't even have energy shields.
In addition, Atriox did take them by surprise. I don't think the outcome would have changed if they knew what they were up against, but it's still something to keep in mind.
Those Spartans were frozen for like 20 some years, and don't have nearly as much experience as John does. John did actually out-wrestle a Brute in First Strike while exhasuted, and with his Mk5 falling apart.
With his Gen 2 Mjolnir, he's much stronger than before. He's comparable to Edward Buck as a Spartan, who in one of the more recent novels pretty easily stomped a Promethean Soldier's head in. That same Soldier can withstand Sticky Grenades, and the Scattershot to some extent.
Atriox is likely still stronger than John, especially with Chainbreaker. But the power gap is nothing like what it was between Atriox and Spartan Team Red.
Master Chief is not an utter bufoon. He's fought many Brutes, and Chieftains. He knows better than to just run head on into H2H combat with a Brute. Especially one as imposing as Atriox.
His standard Arsenal will likely include an Assault Rifle, and some grenades. The rifle is obviously useless, though the Grenades may be able to do some damage to Atriox.
Master Chief will likely scour the battlefield for either a Spartan Laser, or a Rocket launcher. Atriox doesn't have any real ranged options here, aside from pulling Master Chief towards him with Chainbreaker.
But given how one of Red Team survived being slammed by Chainbreaker, I'm pretty confident John will too, and get up.
Besides, I doubt that Atriox could just shurg off a Spartan Laser shot. He'd probably survive it, but would be pretty messed up at the least. This is the same weapon that can do some damage to a Warden Eternal.
Honestly, I'd give it to John High-Difficulty. Due to experience fighting enemies
way stronger than him. And that there are weapons on the battlefield he could potentially use to turn the tide. In addition to being much stronger than any member of Spartan Red Team.
Though it could go the other way as well.
