I have just completed Avalon and I am wondering if azteca is hard
Well I don't think so. It just depends how you look at it.
If you are having trouble with a boss and can't solo it, what do you do? Complain that the boss is impossible on the message boards, or get a friend to help?
Azteca is noticeably harder than Avalon, especially in Twin Giants. I personally don't think Azteca was too hard, because I soloed mobs and some bosses. Some things I would not think twice about soloing like Belloq and dungeons...
It is also about strategy. For the higher health mobs I have to have two feints on each enemy for me to kill, for the higher health bosses I have to have all my blades and all my traps in order to just barely kill the boss. I guess you could say it is "hard" but it is do-able.