Do we have to keep the next match in spoilers still, or has it been enough time?
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So, I only have a very vauge and basic knowledge about Sigma and the Mega Man X games, so pardon my ignorance here. That said... how is this remotely a fair fight? Ultron would wipe the floor with Sigma, with very little effort!
Like, OK, if we restricted this to Ultron pre-Ultron Unlimited, there might be a match up. Just the two of them: two robots that always keep coming back no matter what, two robots that are existential threats to Earth itself.
Actually, wait. I dunno whether minions and such count in this scenario, but we might as well throw them in; Sigma can get all his henchbots and Robot Masters to fight Ultron, and Ultron can get... honestly, nothing really. He's pre-Ultron Unlimited, so he doesn't have armies of drones yet. He has like, occasional human minions and stuff, but I don't think those guys count the same way as Sigma's do. So it's gonna be him versus Sigma and Sigma's whole army.
... and I think Ultron still wins. This is a dude who regularly goes toe-to with the Avengers, after all! Like, jeez, his entire body is made of adamantium! It's not quite indestructible, sure, but the amount of power needed to even dent it... I'm just not sure it exists in the Mega Man X world?
And again, that's Ultron before one of his biggest power upgrades, before he was considered a huge horrific threat, when we was still "merely" one of the Avengers's most dangerous foes, who would destroy the world if not stopped. But what if we include Ultron, post-Ultron Unlimited, but before Annihilation: Conquest? Well, Ultron Unlimited is the story that gave him an army of drones (drones is perhaps not the right word, given that they all share the same conciousness as the primary Ultron). As an aside: Ultron Unlimited is the story where he systematically murdered the entire population of a (fictional) country, just using himself and his drones, and then came extremely close to wiping out the Avengers entirely. Anyways, most of those drones are made of regular-ass metal that exists in the real world, so Sigma and his forces could certainly put up a fight against them (though of course, those regular armored drones will also be doing a lot of damage to the other side as well). About a tenth of those drones are made of secondary adamantium, which is a cheaper, weaker variant of adamantium that is still obscenely durable (for a quick reference: Thor, swinging once at a plate of true adamantium with the full force of his hammer, could make a big dent in it; if he did the exact same against a plate of secondary adamantium, he'd be able to break the whole thing in a single swing. Which maybe doesn't sound impressive, but secondary adamantium is strong enough that it takes Thor-level strength to break!). I'd imagine those secondary adamantium equipped Ultron drones could eventually fall to Sigma and his forces, but each of them are gonna pose a challenge. And of course there's Ultron himself, in his true adamantium body, who is still gonna pose all the same problems as before. But at least the primary Ultron hasn't had any major power increases at this point in time...
But there's more! Oh, is there ever more. What about Ultron after Annihilation: Conquest? That was a big crossover story featuring all the cosmic Marvel characters, and led to the Guardians of the Galaxy forming. It featured a mysterious enemy taking over the entire Kree interstellar empire through mind control and assimilation (kinda like the Borg), and the surprise twist was that the mysterious enemy was in fact Ultron! Whose primary physical body got a huge power boost in this story (in addition to the whole "is now capable of assimilating massive interstellar empires into his being" thing). Annihilation: Conquest (and in pretty much every story Ultron has been in since) firmly established Ultron not just as a threat to Earth, but a truly existential threat to the entire universe, who if not stopped when and where he was, would've easily taken over every inch of the universe, putting every single being under his control. And he has maintained that dire level of seriousness ever since; to take the most recent example of how big of a deal Ultron is right now, the Silver Surfer has been leading Galactus around the universe to eat populated planets in the hopes of slightly slowing down the expansion of Ultron's new empire. Because if the two of them tried to fight Ultron directly, they'd lose. That's a really fucking big deal, and I can't imagine *any* Mega Man character coming even remotely close to any of that!