First things first, to understand how the game works, navigate to the weapons tab in the menu
Select the "Skills" tab, you'll see everything you work with in your playthroughs, perks, weapon classes, field upgrades and ammo modes
Go to the "Weapon classes" tab
You'll then see every class, what you need to prioritize is the first point of Knife class
You may wonder now, how do you get the currency type that allows you to upgrade weapon classes, as well as the rest of the skills tab contents. The best and easiest way is to simply reach wave 10 on any map, immediately after that, unlock the first upgrade for melee class, after that you are free to never bother with this underwhelming class.

This upgrade will help you reach wave 10 way easier, as it says, it replaces gun melee attack which is worthless, with knife quick attack. Knife attack oneshots zombies until wave 10, it also rewards the player with 115 points for every kill.
Everything becomes easier especially in the early waves, because you can manage ammo and points way better, with a bit of luck (2x points and Bonus points power ups) you may Pack a Punch your weapon on wave 6-7 (depending on the map) and after that you need to be literally handicaped to not reach wave 10.

Anyway, you still need to reach wave 10 first to actually unlock melee Knife and turn zombies early waves on baby mode.
Fair point comrade, in this I can't do much other than suggesting you to pick the best class in the game:
Assault Rifles
Not much to say, fast, good magazine size, high damage. After testing the majority of assault rifles in multiple walkthroughs, I have to say that AK47 (no surprise) is your way to go, overall every assault rifle is good, but AK47 just makes things a little easier.
In order to have a clearer view on weapon classes, here's how tier goes imo:
The rest, I won't even bother, they are all bad for regular survival, however they can all shine in Outbreak mode.

With a few words, select AK47 in your loadout (weapons tab -> create a class) and start on either Forsaken or Die Maschine.
Forsaken has wave rush until you enter the first portal (your choice as to when) so it's very easy to reach wave 10.
Die Maschine is an easy map overall due to how open its starting area is and ofc the plane wing camp spot which is kinda broken.

Along with your starting weapon, you also have a field upgrade (special), you fill up its bar by killing zombies, once fully loaded you can unleash it to get out of difficult situations.
The best field upgrades are Aether Shroud, Ring of Fire, Tesla Storm and Frenzied Guard. Here are some pictures that showcase what each of them do:
-Aether Shroud
-Ring of Fire
-Tesla Storm
-Frenzied Guard

However almost all of these upgrades shine after at least the first upgrade, except from one, the Aether Shroud. This is your way to go because it's powerful even without any upgrades. You can ofc try out any of the rest they are still pretty good.

Okay, now that you have a clearer view on how to setup your match, we move further to the guide, forget the camp stragedy from MP or any other stragedy from other Zombie games you may have played. Standing still is your death sentence.
You always need to be on the move and checking your surroundings, do not shoot a zombie as soon as you see it, start running in circles until you have gathered a good amount of zombies, then turn and slaughter all of them.
Always try to find an area that is as open as possible, avoid being inside buildings.

Due to the fact that Zombies mode have so many upgrades and tools, I don't think I have much else to say for the "train" method, it's not something groundbreaking and hard to grasp or even do.

Alright, now we move to the very very good part of the game..Perks. The Holy Trinity of the perks is:
-Juggernog
-Speed cola
-Stamin up
Anything else is irrelevant compared to them EXCEPT from the Tombstone perk but it's useful later on the game, you won't need it on the early waves.

However, YOU SHOULD NOT, rush for perks, instead you should always save points and use the PaP machine to upgrade your weapon at least once, then bother with the perks. The only perk that should, maybe, be used before the PaP upgrade is Juggernog, but if you are patient enough you'll receive it for free.
The thing is that going in higher waves with early purchased perks, may give you some survivability, but without damage you won't be able to survive, you'll end up wasting your points over and over again because you will always be downed, and then you'll need to purchase a few perks again, since some of them are lost when you go down.
And by wasting your points, you won't be able to use PaP fast, so you see where this is going, not points, no damage, no high rounds, no story related tasks.
Perks normally cost 2500-3000 points, but they get higher as you procceed in higher waves, to avoid wasting your points in both early and late waves, you can do certain small easter eggs on some maps to save 2500 points (Juggernog)
In case you do not know how to do the easter eggs, here are some tips:

After you have turned the power on, enter Dark Aether, get PaP machine blueprint, return to reality, install PaP, farm a few points to PaP your weapon once and then shoot some spheres (or w/e they are)
If you are confused, check the minimap shown on my screenshots, right up of it, it will show the name of the area I currently am.

After you have done aether reactors tasks, you return back and power up the PaP machine, then you need to go upstairs and stare at a bunny bear for around 10 seconds.
Here is a clearer view:
-Spawn Spot
(you can see some stairs right to you from that picture)

Once you are teleported into a dark area, you'll need to shoot the bunny bear until it disappears, it will then reappear a little further, you need to do this 3 times and then a chest will spawn with a Legendary Tier/Wonder weapon and a juggernog along with other goodies

NOTE: In this easter egg you need to be a bit lucky in order to get a Juggernog, you'll come accross 3 doors, you can only choose one, if you are lucky enough you'll choose the one with the best reward.
On your way to turn the power on, you'll come across some bunny bear parts, because it is very easy to miss them, I will share the screenshots on where to find them. NOTE, you need to go near the part and press F in order to pick it up, you do not shoot it.
There are 2 doors in the starting area, if you take the left one (left from the spot where you spawn) the part is almost right away (you'll move across the building with a zipline, this is the spot where you land)

NOTE, THIS CAN ONLY BE ACCESSED AFTER YOU TURN THE POWER ON, go back to where the juggernog was, across the street there is a shop, the door is now opened.

These are tools where you either upgrade your weapon's Tier or Armor, or you build defensive/aggressive/special equipment.
Your focus is of course on the Arsenal, upgrade your weapon AT LEAST ONCE before doing anything else.
How to do so you may ask? You upgrade your armor and weapons by collecting salvage dropped from zombies. There are green and blue salvage, each part of green salvage count for 50, and each part of blue salvage count for 10.
In order to upgrade your weapon you'll need

As for armor

Note that you don't have to repair your armor each time, if you have enough salvage to upgrade it to the next level, it will automatically refill your whole bar.

Where the PaP machine is, you'll see 2 areas with light, right and left of the PaP, enter and check for piles of bones etc, they drop green salvage, you can farm a very good amount of them and have your armor and weapon upgraded really fast.

Once you finish the task of the first reactor, instead of wasting points and running into circles, go through that opening that leads to the next reactor
Second reactor
After the final reactor, DO NOT GO UP THE STAIRS, DO NOT WASTE ANY POINTS, just go back the way you came and enter the portal. It can be a bit frustrating, but you need every single point possible.

There aren't any extra tips for Die Maschine that I can think off, simply abuse the plane wing spot, and play around that area in general, it's pretty impossible to die and also take advantage of the dark aether portals, thy can get you out of tough situations.
NOTE: I almost forgot that 10 waves after you turn the power on, the Perk machine appears, it's on the top of the Nach Der Toten building.

Tactical and Lethal equipment cost Green salvage, while Support costs Blue salvage. The best Tactical is the decoy and the Monkey bomb, the best lethal C4 and Semtex and the best support..well everything is overpowered but I prefer the Minigun goes brrr.
You can also build self revive in the support section, but this ability is unlocked at player level 54 (or 55)
However, and even though equipment are important, you should never prioritize them over armor and weapon upgrades, bother with the crafting table AFTER green salvage has become useless for the Arsenal table (which means after you upgrade your armor to Tier 2 and Weapon to Rare)

A few last words for PaP, you may notice an "ammo mod" tab when you upgrade your weapon in PaP machine, the best ammo mod is the explosive due to the massive crowd control it offers and burst damage.

In your playthroughs you'll come across some power ups:
-Instal kill:
Any hit on the regular zombies is lethal, in addition, Elite and Special enemies take increased damage
-2x Points:
Pretty self explanatory, you also get double points upon completing objectives, for example in outbreak mode you get 1000 points for opening some trial chests, if 2x points are active you get double the amount of points
-Bonus Points:
Gives the player +500 points (this can also be increased if 2x points are active)
-Carpenter:
It instantly repair all barriers AND your armor, it also gives you 200 points
-Max Ammo:
Self explanatory, if you see a max ammo power up, do not reload, waste all bullets then pick it up, it will automatically reload your weapon
-Nuke:
Immediately kills enemies within a certain range, also causes massive damage to Elites and Special, once its effect is over and every possible zombie is killed, the player is rewarded with 400 points
-Full power:
This is a very rare power up and you mostly come across it in outbreak/onslaught mode. It fills your Field Upgrade meter to max
-Random perk:
Self explanatory, you won't see this perk in regular survival, I haven't yet
-Fire sale:
It spawns every possible mystery box on the map and reduces the cost of opening it to 10 points

All in all, there is not any deep stragedy on how to use power ups, the only tip I have to give you is to never use Nuke on early waves (or even on later ones) before killing the last zombie. Why you may ask? Because it prevents you from farming points.
If it spawns near the end of the wave, then simply kill every zombie fast and then pick it up, there are a few seconds between rounds where zombies have not yet spawned, this will force Nuke power up to immediately give you an extra 400 points without killing any zombie.

I won't go into detail because there isn't really any other than simply pointing out the weak spot for each Elite/Special.
-Megaton (and its second form):
Weak spot: Head
Elemental weakness: Electricity (dead wire ammo mod)
-Abomination:
Weak Spot: Heads (wait for them to glow, then it's easy)
Elemental weakness: None
-Krasny Soldat:
Weak Spot: Head
Elemental Weakness: Ice, Explosions (Cryofreeze, Shatter Blast ammo mods)
-Mangler:
Weak Spot: Cannon
Elemental Weakness: Fire (Napalm Burst ammo mod)
-Mimic:
Weak Spot: Mouth
Elemental Weakness: Toxicity (Brain Rot ammo mod)
-Disciple:
Weak Spot: Head
Elemental Weakness: None but seems like it's Explosive ammo, pretty much melts it
-Tempest:
Weak Spot: Chest
Elemental Weakness: Toxicity (brain rot ammo mod)

Like I said in the description, this guide will not turn you immediately into a god, you still have to put some effort, but it's not going to take long, you'll eventually reach high rounds with ease and will probably exfil due to being bored.

All in all, that's what I had to share with you guys, I really hope this guide will help anyone that struggle in zombies mode, if anything confuse you make sure to leave a comment down below, this is my second guide ever so mistakes might have been made.
If you liked the guide, rate it, thank you comrades.