Well from my point of view, there are pratical things that the 'Tecs can do to wittle down the technoligical advantage that the Europeans have.
First of all, guns are neither as prevelent now, nore as effective as they become in later centuries. Most European armies are still backed up by large bodies of pikemen and other close combat troops, indeed, the elite royal bodygaurd of the king of France is the Archers.
Firearms are effective only when you gather your man together as a large block and fire at point blank range. Against skirmishing infantry they would be less effective, since targeting is all but impossible with one of these weapons. Also, you get one shot before you get threatened with a charge.
To even things up for the Aztecs, they would be best off with a reliable corp of skirmishing infantry that can harrass and threaten the enemy forces on the feild as well as provide a corp of scouts. A Body of good missle troops comparable to welsh longbowmen, then a large corp of close combat specialists that can close with enemy formations and engage them after they have been suitably wittled down, ideally by breaking up any pikeman that can resist a charge.
In this close up kind of battle, you could imagine a possibly magically enhanced aztec champion causing a lot of damage.
To really do the business, a reasonably powerful water or wind wizard might call down heavy rain or mist, which would furhter impede a European army.