1) Ranged units are secondary. Zombies have very high pierce resistance, which makes them take little damage from ranged attacks, don´t march in closed formations, which makes them hard to hit, and even fire or fiery arrows don´t do that much damage to them. They can be a decent support, especially against the undead ogres, if you put them on the corner of your battlements around which most zombies walk (leave one of your gates open, so they will be lured there, and you only have 1 place you have to defend). Without fire/fiery arrows, archers are worse than useless against zombies however, so if you´re gonna use archers, get those asap. Or you can use the dwarven riflemen, who of course don´t have flaming ammunition, but on the other hand, do significantly more piercing damage, and have a better accuracy against moving targets (I think at least).

2) Since ranged units are secondary, never get more than, say, 15 battalions of them or so. I would probably recomment even fewer, more like 10 maybe. But maybe even 20 or more could work if you get dwarven riflemen and use them actively to support your troops (not just place them on a key portion of your fortification).

3) In ranged troops, improve damage most of all, although I personally like to give a little regen to every battalion, especially those who don´t have an active heal ability, and range can also help quite a bit, the rest I find mostly useless, however (in zombie survival at least).

3) Focus on melee units. Units with slashing attack, ideally (that´s the one with the sword icon, same as zombies have). Because zombies, especially in the earlier waves, have low slashing resistance. And even more importantly, choose a unit with a high slashing armor. I would recommend mounted grandmasters as elves, as they have 86% slash resistance when upgraded, are fast, have slashing dmg, can heal, and can lvl up to lvl 10. Also, in case you keep wondering like I have for quite a while - the armor types are noted in the unit intefrace in the following order: slashing/impact/piercing.

4) Get the Protector the sooner the better, and improve his stamina. Use him to heal all units around him.

5) Once you get to wave 6 or so, it will get harder and you should probably use a greater amount (cca 10) of battalions to engage zombies in any given spot, healing them all if possible with their ability and the Protector´s as well. Use them wisely though, as stamina regenerates slower than how fast you use it up.

6) Keep one gate permanently open and the other permanently closed. Zombies (watch out for occasional exceptions though) will be drawn towards the open gate, which will leave you focusing on defending a single spot (defend it from further away though, don´t just block your opened gate with your troops, that would be a ♥♥♥♥♥♥♥ disaster).

7) Exceptions to the above meantioned rule most notably occur when you garrison your archers or riflemen on your walls - then zombies sometimes try to tear the walls down, so I don´t recommend mounting the portions of the wall which aren´t also protected by melee units, with archers. There is nothing worse than fighting on two fronts in the middle of the map, and finding out a few zombies slipped by, torn down a portion of the wall or a gate, and slaughtered half or your workers.

8) Try to engage zombies as much as you can during the day, when they are less aggressive, move slower, often don´t react to even being attacked quickly enough to harm your troops. Push them as far as you can, even to the edge of their graveyards (you will have to fight on 2 fronts if you play singleplayer, or just one, if you play coop). Inversely, when the night comes, fall back and only engage when they get you cornered at your city´s fortification. This tactic can save you a lot of troops.

9) That being said however, NEVER push them all the way back to their graveyards - if your units, even heroes, enter the graveyard, they will get a debuff - a plague of sorts if you will - that will do massive damage to them, and also drain stamina.

10) It helps a great deal to use cards, even when playing singleplayer. I would definitely recommend getting the 4 mounted grandmasters card, as it will cover you for like the first 2 waves without a problem (if you´re careful enough). You can use the spare time and resources to develop your economy extra fast. As for other cards, I would recommend resource cards, but others can definitely work as well, depending on your overall strategy and focus. Don´t be afraid to try new stuff :P

11) Don´t let your battalions die. Ok, that might sound like a moronic tip, but you can absolutely do that. Just withdraw every battalion that reached the red hp numbers, (or maybe rather each that´s gone to >25% hp or so), and have them rest and heal by your walls. while rested battalions do the fighting.

12) If you use ranged troops, always set them to the most aggressive, offensive stance or other setting you can. They don´t get attacked, if they do, you´re doing it wrong (mistakes happen though, so I recommend giving every ranged unit +1 regen asap). For elves, that means setting them on "wind" stance (while I personally prefer setting my melee troops on "earth" stance, which is more defensive - but I don´t know, maybe "wind" is better even for them, especially if they aren´t supported by ranged troops, so don´t hesitate to try that out and let me know in the comments how it went :) ).

13) When upgrading your melee units, you should always give them as much slashing resistance as possible, when you max out, put it in regeneration, health, and damage (I do it in this order, and it works well for me), obviously there is no reason to upgrade piercing and crushing resistances, as zombies all have slashing dmg. Putting points into stamina also doesn´t seem necessary for most troops, the Protector and other heroes being exceptions. Putting 1 point in stamina for mounted grandmasters, mounted knights, and other similar troops with an active self-heal ability might be a good thing though, I will try it out next time I play.

14) When you get farther into the game, you might want to build a second building from which you recruit the unit that makes up the bulk of your army. In wave 8 and higher, losses can be significant in each wave, and you might not be able to supply your ranks with new troops quickly enough to hold the line. Especially when the wave arrives just as the sun sets (see the first point).

15) Oh, and obviously, max out those buildings´ lvls as soon as you can, in fact, unless you have to, I wouldn´t recruit any troops before you get to lvl 5 with a building. Four points make a huge difference, in zombie survival in particular.