Infantry?!

No, no, no, no. Bring cavalry. Light, heavy, medium... anything with a lance. Half a stack of cavalry with shields can wipe out the entire Aztec army. A full stack if they aren't carrying shields, as you'll suffer more losses. Hire the local mercs for your sieges if necessary.

You can consistently get 10 or 15-1 kill ratios with cavalry stacks against the Aztecs.