Any survival tips for Aztecs ?

Just remember that if the Hordes die, it doesn't count. But if the Hordes (or any of the other elements) kill enemy, that does count.

I've played a game against Aztecs where I killed 9 elements and still lost.

Which means that your opponent will target your Psloi and Warband as the easiest “meaningful” kills - so protect them.

There doesn't seem to be too much in the way of tactical advice for horde-heavy armies, so here are a few suggestions for fighting IV/68e Spanish with IV/63 Aztec. Note that they might not apply to other match-ups, and some players may disagree with some or all of them, but they may give you some ideas.

Both armies are Aggression 3, so you have a 50% chance of being the defender. If you get the chance, set terrain that is favourable to you and unfavourable to the Spanish. His army is severely outnumbered by troops that can exploit difficult terrain, so use that advantage. Arable terrain gives you a good selection for this.

Your army has no effective good-going counter for 3Kn, so avoid fighting knights in good going, or if you absolutely must, sacrifice a horde. Note that the Spanish jinetes are capable of enveloping one or both of your flanks unless you are very careful with your terrain placement, camp location and deployment. I like to place terrain fairly close to the board centre so it complicates my opponent's command and control. Where possible, use threat zones from troops in bad going to cover gaps. Also keep in mind that your general's command distance in bad going is only 4BW. Keep your army fairly close together and use your general's mobility to extend his command distance by moving him where he needs to be.

When using armies capable of exploiting difficult terrain, I like to take an edifice, two difficult hills and a road, with the area terrain being on the larger size if possible. With a little luck, this will limit the offensive potential of his mounted troops and (hopefully) the hills will give you a slope or two to defend on. The road ensures that you can limit the attacker's choice of sides to the two you select, instead of the usual 4. If possible, place it so that it can be defended from within difficult terrain, or through terrain you intend to occupy. I tend to avoid using rivers because they can slow games down so much that a draw becomes inevitable.

If you are the attacker, you may only have one small piece of terrain to exploit. Always occupy it with your 3Wb and some support. That should make him think twice before he sends in his fast blades. He only has 4 elements that are effective in difficult terrain, so you can easily achieve a numerical advantage in that area. Only his Psiloi can kill your psiloi in bad going (unless they are pinned), so they are extremely valuable to both players. Kill his if you can. Also, remember the double psiloi move, not just in the first turn, but later as well. You can use it to redeploy psiloi quickly in and around terrain ... but so can he, so be aware of the possibilities. Unlike him, with adequate pips you can move all your troop types through bad going reasonably quickly. If he makes the mistake of placing his camp near some, there may be an option for some of your fast troops to take it while he is occupied elsewhere.

Using hordes well is no easy task, but IMHO 5Hd are the better option and can give you some surprising outcomes. You only need to kill 4 of his elements (or equivalent) to win. His worst case is that he may have to kill 10 of yours! Exploit his tendency to pursue and don't worry about losing a horde if it will give you a positional advantage. Sometimes (ok, rarely) a 6:1 might even save you from being annihilated. Perhaps others here can give you some tips for the other potential match-ups. Good luck!

They look fab anyway

Win or lose, yours will be one of the finest looking armies out there.  Well done.  Very spiff.  Not much advice to add to the comments already posited.  Hordes are often demeaned for being such PIP-hogs, and, it's true; they are.  But, once a Horde is "stuck in" against an opponent, it can also get any number of "free moves" too.  (So long as it wins in Close Combat, of course.)

Essex miniatures.
I have bought a ready made army box but was a bit disappointed by the lack of diversity of it. I chose the aztec army for its colorful suits...unfortunatly, it contained mainly 3 poses bare-chested/quilted jacket men (6x5Hd + 6x3 Aux).
If you need one, I recommand you to select the minis set by set.