The correct answer is - forming alliances with the Aztecs' enemies.
There were multiple factors as to why Hernan Cortes managed to conquer the most powerful empire in Mezoamerica, the Aztec Empire, and one of the biggest factors was the wise tactical forming of alliances with the enemies of the Aztecs. The Spanish conquistadors on themselves, would have had almost an impossible task to conquer the Aztecs, but they played wisely, and managed to get on their side the people that despised the Aztecs, and used their knowledge of the military style, as well as of the surrounding terrain.
The Aztecs were a great civilization, but one thing in which they were very bad, was the ruling over other people, as they did it with aggression, and that made pretty much all the people under their rule to hate them. The Spanish used this in their advantage, and managed to beat the Aztecs with relative ease.
