After experimenting with HRE vs. Aztec custom battles, here are some possible helpers:

1) DON'T bother with sword or spear infantry. They'll be destroyed by the Aztecs in melee.

2) DO use lots of of pikemen and, perhaps, halberdiers. In the case of Spain, you're lucky that you have Tercios.

3) DON'T use heavy cavalry unless you can ensure they'll get a good charge, as they also tend to fall apart in melee

4) DO use missile cavalry, especially those that can melee reasonably well (like HRE Reiters); you can charge them in after you weaken the enemy with barrages. Those that don't (like, I suspect, Mounted Crossbowmen), be prepared to micromanage them.

5) DON'T use exclusively gunpowder infantry.

6) DO use Musketeers AND Pavise Crossbowmen in concert. The former are to scare the Aztecs, while the latter are there for sheer killing prowess (and survivability, as your supply lines are effectively nonexistent in the New World).

7) DON'T expect artillery to be a panacea, and don't bother with really big artillery pieces either.

8) DO bring artillery like Serpentines and try and concentrate them on the enemy general; and/or some Ribaults to strengthen your flanks.


Hope these help!