Just like in Payday, crouching while having a human NPC facing you would has a slower alert gain.
While being seen in close distance will trigger full group alert.
You can let it goes beyond yellow make them come over the "suspicious spot",
then you can either sneak past them,
or gain enough distance to build red meter a bit behind a cover, and kill the enemy who is now separated from his group.

The "Sprinting crouch jump" in Payday works here too.

Running causes a huge range of noise which can alert walkers in all direction, to roughly "2 cars apart" from you.

Even during a horde walking can reduce walkers chasing you for a very huge number, and with enough corner turns / walls and climbs some of those chasing walkers will lost track.

On target melee attack, both light & heavy causes noise to walkers (or perhaps human too) about 1 car wide.
And for some reason, during a horde, killing 2-3 walkers is enough to draw a large group of walkers from 360, which is much wider than no horde is present.
Choose wisely when to kill, and have a plan on where to escape and eventually reach your destination with thinned horde.

During a horde, even at horde tier 3, producing noise on purpose at one spot then silently move away from the position of the noise produced can often lure walkers away from flooding your objective location.

Slamming your melee weapon on enviromental stuff doesn't give noise.
Eg. Hitting a metal surface truck when you're on top of it doesn't help drawing more walkers.

Landing from a jump produces noise.

Your flashlight increases walker's detection distance greatly, and the angle for a bit.
It has no effect on a walker with his back facing you.
[Pointed out by Couch]

Walkers and Human do not react to lights.

Walker
During takedown other walkers cannot grab you, it lasts a short while after the takedown animation is completed. There's enough time to freely use a light attack or shove before a walker can grab you.

Probably known to many, you should stab when a single walker is grabbing you.
Unless in very hard or above, you'll get less than 1 bar of hp damage and can regen them back.
It also gives a push to the newly killed walker, creating a CC longer and wider than a shove.

If you've already stabbed 4 times before a 2nd walker grabs you, stabbing the 5th hit will always trigger the kill and you'll be fine.

Human
A human enemy can only aim, discover you if your view to his head can produce a perfect straight line, same as in Payday.

Human enemy will try to flank you if you stay in a position for too long, keep changing cover from time to time can fool the human enemy to aim at your original position if the change was not spotted by them.
Even a simple "left side of door to right side" change, if unspotted, the AI charging to the door will look towards the left side 1st, conserving yourself a bit of hp and gets a free hit.

AI's do need a bit of time to "aim" before firing at you, they're just super quick at spotting their target (you) is within their line of fire.
Use cover, hold aim and slide slowly to eliminate possible hiding spots bit by bit,
you'll have the upperhand to shoot first when an AI moves out from cover.

Environment
Barricading a door adds a 3 hit protection to the door, and can be repeated any amount of time. If playing in a group, 1 player can dedicate to block a door for a long time by repeating barricade, once the 3 wood planks are gone, open the door to shove away the walkers, close it up and barricade again.

A higher ground without slope for walkers to approach is a good spot for luring walkers away from your objective locations. Create some noises there, sprint jump to hop over them when you're ready to go.
Note that tiny higher grounds are still very vulunable to Bloaters.

Each character's primary skill uses 3 of the 4 materials, its best to use a third skill which uses the remaining 4th material if you priorize looting.

On the other hand, each primary skill has a material that requires 4 of it to craft, while looting with 3rd skill, it either gives you a specific kind of material, supplies or both.
Which means you can use the corresponding 3rd skill to maximaze the material you get for your primary skill.

Shiv can be used to shoot at a wired trap, I know we all had died from explosive traps by melee hitting the wire so thats why.

3rd skill can be used to repair progress blocking fixing events.
[Provided by We Butter The Bread With Butter, Space Viking, Ethanos Gaming]
eg, 1 Electronic Kit subs for 1 fuse. 1 Mechanical kit for 1 gear, 1 Chem kit for 1 battery.

You can aim with your weapon from a far away position to figure out whether you or your team can unlock a shortcut by using callout. So you don't have to dive into a horde and swim within it just to find out you're heading to a deadend.

Heavy
Hemlet can be knocked off, thus:

Possible 1 shots with headshots after using Grant's Marked For Death.
(Hold callout until icon on enemy's head)

There's also a crack on the back of his armor, it needs a melee or few shots to break and reveal Heavy's flesh.

Can shoot legs to CC, he'll kneel down and become wide open for a moment.
Takes hell a lot of ammo to do this though.
[By Colelyoe2 & Space Viking]

Can cause openings if being surrounded by walkers, he'll keep using shield bash and opens up his head for shots.

Also uses the same "Targeted" icon when you're being targeted by a sniper.

During a horde, especially tier 3, when needed to travel around objectives,
not killing him is often an option, he's the type of tank you want to party with in every MMORPGs.
Once occupied by walkers he ignores you until all walkers around him are killed.

Bloater
Don't bother shooting him. We're so trained to 1 shot L4D Boomers sigh.

Instant death on shove, combining with the alert tips above just take your time, approach from back and give him a push. Still requires sprinting out to avoid AoE damage.
Timing is much more strict when you're carrying a weapon case / objective item.

If being noticed better not to shove but just sprint out after entering a close distance.
Close enough to hear him burp is a good sign to immediately turn 180 and sprint out.

Hordes in this game is the exact reverse of L4D and Vermintide,
you're forced to keep moving instead of staying at a spot to clear the whole horde, due to the quick respawn of killed Walkers.

It is in fact possible to stall a point from being overrun by Walkers for a very long time using weapon characteristics, but the number one rule during a T3 horde to hold a spot is, do not kill the walkers.

The reason for not killing them is, it doesn't matter how quick you can kill, it won't stop their forward movement by killing, and in T3 I call them "Instant refill Walkers", they simply respawn at the same time.

Instead, if you're able to apply various effects to the frontline of the horde, no Walkers will be respawned and the frontline being CCed will slow down the entire horde using that path.

Another benefit for not killing the frontline Walkers in a choke point is, killing them simply allows the 2nd row to approach, who are ready for a grab.

If you're in a coordinated group, the following will greatly change your impression about different weapons greatly, especially after understanding killing is not the best option during a horde.

Weapon of Choice: Axe, Machete
Characters: Bridger, Maya

The purpose of dismemberment is to cut down a Walker's leg, causing their movement speed to be greatly reduced.

When used in a chokepoint combined with shoving, the Walkers indeed choke from there.
The crawling Walkers up front will block those behind them from moving at walking speed, while having shoves from time to time can further reduce their forward movement.

This here is a story of what was done with dismemberment choke point defending:

I was once in a Very Hard game in Hell or High Water, our group already reached T3 before the forklift gate.

The back gate on the other side of the forklift was opened, and I was the one using Bridger with axe (yes the weapon everyone thinks it's bad), defending the backgate while the 3 others had all parts and fixing the forklift on the other side.

Using the above method, I was completely drained out of stamina for 2 times and the Walkers had not went past the crates yet, and that's the time when I heard "Molotov!" "Frag Mine!" from the other side... And BOOM!, we're already rushing our way on the stairs inside the gate.

Crawling Walkers are unable to attack the door, it should help the barricade rinse and repeat method further.

A crawling Walker is a good shoving target, not only it bounces further away, it crawls back to you slowly allowing it to block Walkers behind and letting you recover a bit stamina.

Axe is probably the best choice for this purpose because of its cleave heavy attack, very notable when being used by Bridger.

Machete is more close to a "Jack of 2 trades" between a quick 1 hit kill and heavy attack single target dismemberment. I couldn't figure out a method to hold a choke point effectively with this weapon.


Weapon of Choice: Baseball bat, Quarterstaff
Characters: Aiden, Grant

The stun animation of a Walker from a heavy hit with Basebat by Aiden can sometimes become extra long, it also bounces back Walkers. (Bounce back is not obvious when there are already a huge load of Walkers behind your target)

With Aiden using Baseball bat, his stamina consumpsion is reduced to a point which he can perform a heavy attack, and restore 50% of the used stamina then go for another heavy attack.

The heavy attack of the Baseball bat used on Aiden is a stronger, less stamina version of shove.
Stalling effect is huge on an already broken door or choke point.

(I don't have my Grant's 4th melee skill unlocked yet, the following are merely my observation during leveling up Grant)

Quarterstaff as shown by the stat graph has fewer stagging power than a Baseball Bat, but it's heavy attack being a 2 hit is really the best among all weapons to "swim" through a horde.

Swimming through a horde is risky though, a Bloater can totally murder you if you happened to swim to him.

It still does well in choke points to stall.

Stagger VS Human

Very effective in maintaining stealth, the stagger works like shove which can stop the patrolling human who discovered you from alerting his team.

A successful stagger will cause the human to fall down just like being shoved,
at that time you can combo that into a light attack, then shove / heavy attack as it's about to finish his standing up animation.

I managed to kill 2 AIs right in front of me during red meter without alerting other 3 patrolling AI who're also in red meter, coming to my position on another path with the above method.

Pickaxe / Machete
They are both good for quickly killing a patrolling AI who has spotted you in point blank range.
Their choke point defend power is really bad, especially pickaxe.
With Machete it still functions like an Axe if the choke point is a door. (1 Walker wide).

They're both the quickest weapon to knockoff a Heavy's helmet.

Axe / Baseball Bat / Quarterstaff

These 3 are the best choke point defence weapons.

Dismemberment from Axe compensate its high stamina and slower animation,
it also provides long term benefit in a horde by leaving crawling Walkers here and there,
they're easy to evade and usually won't cause too much trouble even if you get caught.

Baseball Bat is the most stable and effective choke point defender, there're times a horde simply can't move an inch at a chokepoint.
It's also good to keep patrolling AI to alert others when being spotted.

For Quarterstaff, I don't have a high level Grant, nor a high level weapon yet.
But it certainly is the easiest weapon to kill a patrolling AI when it spots you,
if Marked for Death is used it's almost 100% 1 body hit kill.

First off, you can't really do this in pug coop mode,
it's possible to stealth assassnate with a coordinated group too.

When a circle meter appears above an AI, it means you and him has a direct eye contact to each others.
Upon reaching a full yellow circle, it will spread the yellow circle to AI's close to the initial AI,
and start patrolling to the spot where the yellow circle was first accumulated.

Note that within a specific close range, the meter build up mechanics will be ignored and AI's in the whole area will go full hostile. I will refer this range as "exposed range".
(Of course no living being will become "suspicious" if a hostile is standing right in front of its face)

More direct eye contact will go beyond the yellow meter will accuminate the red meter.
I'm not 100% sure what is changed but I suspect the exposesd range is longer than usual state.

The AI will keep patrolling to the spot where the last eye contact was made,
his field of view is almost directly straight towards the spot it's checking.
It allows baiting an AI to a spot, without noticing you hiding in a corner along its path, thus dealing a takedown to the AI on a isolated zone.

- After Takingdown or silent sniping every isolated human, there're still groups of them having a 360 detection to your destination.
- Look around, find two covers close to each others.
- Stand and trigger a full yellow meter, at this point 3 or more of them will go full yellow as well, and start walking to the position which the full yellow meter has reached.

This point onward lots of practice and trial & error is probably needed.

- Move away and get behind cover A. You can sprint as long as the red meter is not full.
- To your initial target, he should have more red meter than the rest of his group, and will search for you longer allow him to serparate from his group.
- Move to cover B, this is to give you a better chance that the RNG search after the AI reaches the last "suspicious point" will not spot you as easily.
- You may and to steer your angle a bit to avoid being seen, align cover to hide his head on your screen.
- After hearing "Back to base", "Whatever.." etc., it's good to follow the guy and takendown.

Note that the above example is just a scenario, which means your "finishing move" doesn't have to be that "follow and takedown". In fact in some cases I'd go for a shove > heavy > light attack on a standing still AI because I would have locked into takedown animation (uncrouched) and cannot avoid being spotted.

There're times when things not going perfectly to your plan, and you know you'll be ♥♥♥♥ if you move away from your cover or do nothing.
It's not over yet.

You can still try to kill the AI who just reached the corner of your cover quickly.
By quickly, it goes like these:
1 heavy attack to the head.
1 heavy attack + 1 light attack.
Spamming your silenced weapon quickly.
Stagger or shove to cause knockdown, attack while AI standing up but make sure another shove or stagger should hit before he finishes his stand up animation.

The loud combat music will pop up a bit and stop after 1 second.
The enemy spotted enemy will also yell like the entire world can be heard, but don't panic.

Note that a 3-4 light attack kill will gurantee the AI alerts his group and goes full group combat mode.

After an AI spotted you, shoving is able to reset the time it notify other AIs.
It even doesn't matter if the AI had fired his gun on you, it only matters if you had shoved or staggered, causing a knockdown before a short window of time after it spotted you.

Filling a sniper's meter to yellow will cause a group of non-sniper AI's become yellow, and they will patrol to the sniper's last seen position of yours.
This can be used as a tool to lure AI's to a better cover spot you have.

Sometimes a pair of "chatting buddies" will no longer stay at their spot to chat after returning from patrol.

So we now have knowledge about stagger and dismemberment, as well as some basic idea on how different stealth kills can be done,
lets take a look at the primary skills and see what effects when they're used by their owners / vanilla mode as secondary skill:

Smoke Grenade
Vanilla:
Enables takedown to enemies inside the smoke area.
Enemies will stop attacking if you're within the smoke.
2nd quickest detonation time among 3 grenades, still suitable as an emergency protective tool.

Owner:
Longer duration, more range of smoke.
Adds slow, stun, to those caught in the smoke.
Marks enemy for 30s, marked enemy takes 20% more damage
Gunshots going through the smoke has 80% chance to miss.

Smoke grenade itself is nice enough when used as vanilla when your stealth plan is about to be ruined, it often saves you from being surrounded a huge number of patrols and allow you to kill them quickly.

With its owner it can be used more aggresively, such as quick killing a Heavy while allowing an escape to a cover with the 80% miss chance.
The stun effect also makes killing Heavy easier because Heavy in the vanilla version will still spin around, facing you with his shield on.

Flashbang
Vanilla:
Stuns enemy within explosion range.
Longest detonation time, not too good for emergency use.
Drops your FPS to 10 for the duration of time before it detonates, if you didn't tweak your display settings well.

Owner:
Draws enemy to the flashbang before it detonates.
Blind and stun.

Because of the long detonation time and no lure effect, vanilla flashbang has very limited use.
On its owner, it can be used as a really powerful tool to kill over 5 human enemies within seconds, due to the lure and stun effect.

Molotov, Frag Mine
WIP - needs a fully unlocked skill and test the difference.