For the sake of argument, let's argue under the basis that he can't block poisons, then. What's your argument for Alex winning?
Mercer gasses.
And since teenage Gojo can’t block it, he gets caught.
And since teenage Gojo does not appear to have Domain Expansion, he can’t use Unlimited Void, so he has no way to stun Mercer enough to land Hollow Purple on Mercer.
Teenage Gojo also doesn’t appear to possess Teleportation like his adult self does either, so he has no way to avoid it even with speed being equal.
Even if Gojo tries to use Blue to pull the gasses (which teenage Gojo lacks the scans to prove that it’s in-character for him to do that against gas attacks), Mercer has Infinite stamina whereas Gojo doesn’t (and considering even adult Gojo was unable to spam Domain Expansion more than 5-6 times without suffering from brain damage or whatever, along with the fact that there were some statements stating that Gojo's Cursed Energy was running out against Sukuna, teenage Gojo's Cursed Energy is definitely not infinite), so Mercer can just keep trying to infect Gojo until Gojo gets worn down enough for the gasses to land on Gojo.
Due to teenage Gojo only ever having shown to use smaller-sized Hollow Purple to blast a hole in Toji's torso (unlike his adult self who can use a much larger-sized Hollow Purple to obliterate entire surroundings, or practically obliterate an entire city like what Post-Prison Realm Gojo does), teenage Gojo wouldn’t be able to use a Hollow Purple larger than that due to lacking the feats his adult self does… Which means Mercer has a much higher chance of surviving that Toji-sized torso hole caused by Hollow Purple compared to adult Gojo's Hollow Purple - so Mercer just regenerates and gets back to haxing Gojo with the gasses.
And with Mercer having combat applicable Low-High Regeneration, with that Mid-High regeneration that takes hours when regenerating from incineration-level attacks, Gojo's Blue and Red is basically ineffective against Mercer's regeneration that easily works in a few seconds.
Or in short, Gojo gets gassed by Prototype 2 Mercer (who hates Humanity and would try to infect any humans as soon as possible without PiS holding him back), and gets infected.
And while Gojo's Limitless may allow him to manipulate Space at the atomic level, there’s no scans that suggests Gojo's own RCT works anywhere near that level, so it’s very likely the Blacklight Virus molecular-level infection would keep Gojo being a mindless infected - and considering the fact that the Blacklight Virus was shown to immediately convert a victim into an Infected in just 1 second, Gojo's RCT wouldn’t be fast enough to counter that as well (it took him a few Manga pages of fighting for him to regrow an arm if I recall correctly).
And considering the Blacklight Virus is designed to be superior to the Redlight Virus, which has explicit statement of altering the structure of the brain (the thing that also causes Madness Manipulation), teenage Gojo is even more screwed as the brain is what allows a Sorcerer to control their Cursed Energy.
So yeah, Prototype 2 Mercer just gasses teenage Gojo, and infects him.
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