I believe the idea is so that they are completely destroyed in order for the achievement to unlock. Have you had any success with this yet?
Without ruining your ironman save. What you could is to try and give them a county. By doing so you should then be able to declare war on them, destroying their armies before ending the war, and thus triggering the "has_regiments = no" trigger in order to get the "set_global_flag = aztecs_defeated" flag.
I'll see if I can get some answers on whatever the units should disappear after a while though.
Without ruining your ironman save. What you could is to try and give them a county. By doing so you should then be able to declare war on them, destroying their armies before ending the war, and thus triggering the "has_regiments = no" trigger in order to get the "set_global_flag = aztecs_defeated" flag.
I'll see if I can get some answers on whatever the units should disappear after a while though.