this is for multiplayer only. aztecs can auto-heal, but only if they win. with the new patch you can retreat and the opponent wont upgrade, but you can still take out 1 or 2 units of the army. so if you have 2 armies against and aztec's one army, you can just attack, kill one unit, retreat, then attack with the other unit and probably win, otherwise just retreat again. This strategy will probably only work once on a defensive unit, but that may be enough to finish them off.