Let's assume that many of the peoples of the Americas have been convinced of these (and only these) facts:
- People are going to come on large ships from the East.
- If they are able to send a message home, their people will come to kill you, enslave you, and give you plague.
- They are the Borg. You can't beat them. Not even you, Aztecs.
This might lead them to cooperate in creating settlements along the coast specifically to keep watch for ships, even forming garrisons in less hospitable areas that would usually be sparsely populated. This wouldn't last forever -- eventually they'll get an "explorer" who's bloodthirsty enough to kill them first (alternately, eventually some subgroup is going to defect and try to ally with the Europeans to get an advantage over their rivals). On the other hand, if everyone who tries sailing West is never heard from again, that will probably slow down the rate at which Europeans keep throwing themselves at the problem. Let's say it buys the Americans 100 years. What are they going to do?
First of all, they're going to study the artifacts they recover from the explorers. They'll learn 1) how to make firearms and other European weapons, and 2) "Oh ****, we're going to need a lot more metal." This might lead them to rapidly push toward industrialization (and for groups that hadn't already transitioned to a more sedentary, agricultural society, this would push that to happen quickly). This is a massive change to everyone's way of life, but they're aware that the alternative is genocide.
Of course, this doesn't turn them into angels. Some of them will probably use their technology to kick the butts of all the people farther to the West. But that might be beneficial in terms of getting everyone working together on the resistance.
So, when Europeans eventually hear about what's over there, they might find a land more like their own, with densely-populated cities, specialized militaries, and no willingness to put up with their BS. The Americans don't have to be able to win a shooting war with all of Europe, just able to work their way into the existing system of trade and alliances rather than getting overrun.
Unfortunately, this would do absolutely nothing to help the Americans against the real European weapon: plague. I've seen estimated mortality rates for smallpox and influenza that are as high as 90%. High population density would make it even worse. They wouldn't even be able to use quarantine for arriving Europeans, because some would be asymptomatic carriers. I can't think of any way that you could have a "soft" introduction of European diseases to the Americas, allowing the native population to acquire the resistance that Europeans developed over thousands of years. There are low-tech vaccination methods, but allowing them to know about that in advance goes outside the scope of your scenario. To the extent that the ability to survive these diseases is genetic, enthusiastic interbreeding might help... but it would still put the existing population at a huge disadvantage when they first made contact.
In the end you might have the same result as we had in real life, with the exception that Europeans would have a harder time rewriting history as a narrative of cultural superiority and colonization.
... okay, I wanted to believe that last part, but if that happened we'd probably just rewrite it as a narrative of biological superiority instead.