From head on. We're talking about some of the best guerrilla warfare specialists. Many Native Americans liked to hide in trees to fire ranged weaponry. Not to mention that vikings shields, well, sucked. Wooden bucklers don't make for effective shield walls against projectiles. You really need tower shields, at the least large kite shields.
I don't doubt that a proper viking offensive wouldn't have been successful though, just hard. They had some of the best infantry in the world, and infantry would have been infinitely more useful than mounted warfare when they shit hit the fan, another reason they would have been more successful than the Spanish. I only think they would have had to worry about supply lines. The rivers would only take them so far, at least against the Inca, they could have landed in some of the Aztec's most important cities with almost no resistance. But the Aztec would ingeniously trapped their ships in, breaking the supply line. Not that the tactic worked for the Chinese against the British, but it would have been annoying to say the least.
If anything though, the vikings would have become good friends and trading partners with the Aztec. They would have certainly respected their fighting prowess, especially the macuahuitl, and if they were pre-christian they might not have been too upset about sacrifices, only the act of removing an enemies right to die in battle.
But all of that asside, I just picture a berserk viking running naked down the streets of an Aztec city butchering everything in his path, with a few spears stuck into one side...
"Hey! Could you do that on the temple instead? Shame to waste all that blood."
I like to imagine that the berserker accounts we hear about are really just lost bets and too much booze before a battle...
"Remember, you have to go into battle naked, and drink our strongest mead until I take the mug out of your hands, or the enemies get here first."