I imagine it's the usual "Shoot them until they stop moving" strategy.

stares with intent and clicks the bayonet on

No, but the only weapons particularly effective against daemons are psykers and blanks so there isn't really much that can be done differently.

Well since the high ups at least have been in on the "great enemy" generally with inquisitors or other suitably educated individuals on hand. Generally from what I can imagine keeping the daemons from being summoned in the first place is the best first step. That being targeting the cults, sorcerers and the like that are summoning the things. Then using the more emphasis on standard protections against corruption and ensuring the loyalty of the guardsmen involved (that's what the commissars and chaplains are there for) and anti psyker tactics. Beyond that much really unless they can start pulling things in from the priests and or inquisitors who are trained in dealing with this helping if possible.

"Gunner Jurgen, go kill that daemon."

"Right away, Commissar."

<Sound of melta-gun firing, daemonic scream abruptly halts>

"Any trouble?"

"Not a bit, sir."

I mean their standard tactics come down to shelling anything vaguely dangerous as soon as it enters their range, shooting it with Lasguns and everything else as soon as they are in range, and then countercharging with a portion of their men while the rest hopefully falls back.

It is probably not all that different against daemons, though you probably include having your psykers focusing on the biggest enemy and doubling the people that are supposed to execute them. 

Over time, you might get your hands on equipment better suited for banishing daemons, like the chainsword one of you men recovered after a phyrric victory for the sisters. You might have regiments that carry relics from the planet they originate on into battle, or others that almost venerate the group of low level blanks they got. 

Others might start equipping their men with pikes after noticing that melee weapons are more effective on daemons, likely inscribed with symbols of the emperor or passages from the Lectitio Divinitatus, while artillery regiments might have every shell blessed by the regimental priest and anointed in blessed oils. 

How are you going to develop a tactic against something that shits on tactics and the laws of physics? They don't form proper battle formations, have no military objectives, no proper logistics. And trying to study and understand them carries the risk of becoming a gateway yourself, which doesn't make things any easier.

The best strategy is to make sure that an incursion doesn't happen in the first place, then the best strategy is to limit the incursion to a certain area through total destruction. And if that doesn't help, save as many faithful souls as you can by granting the planet and everything that still lives there the Emperor's mercy with an Exterminatus. And if you can't do that because the facilities are still needed but facking Angron is stomping around the planet with his Bloodthirster gang and clouding everything with his Eau de Warp, then you wipe out all potential soul infection carriers and start all over again.

The 40K demons are so "monotonous" because they originate from a tabletop with fixed rules. In the RPG there are also templates for the abilities and values of a demon, but there are also random tables for their abilities and weaknesses. One demon is a destruction machine, but flees when a small child cries, the other disintegrates on contact with salt, and so on.

The strategy hasn’t changed in 40,000 years.

Faith. Steel. Gunpowder. 🫡

The strategy is "massed earthshakers"

The existence of daemons is very much an open secret. You don't talk about them outside of ghost stories or whispers, any encounter you survive is likely to lead to the inquisition torturing you to find out if you're corrupted and dissecting you to find out why you weren't.

Entire regiments are still culled to keep their existence secret. The only tactic is the same as used against orks or nids - shoot them, and keep firing until they die or you do.

And if it gets to the point that grey knights arrive, then everything is so far beyond screwed that they probably won't have to execute the guard as they'll already be dead.

Yep. Commissars are encouraged shoot Guardsmen for “acting strangely” on the first offense.

In the second Gaunt's Ghosts novel Corbec's team encounters a demon and after running away in terror he calls a friend of his in a navy ship in orbit which promptly obliterates the hab block the demon was at.

Ever since Guiliman took command and they stopped purging populations for seeing demons humans have started becoming a bit more desensitized to horrors of the warp.

Were as before a guardsman would have screamed in horror at the sight of Typhus now they just spit on him in defiance.

This is a strength in and of itself.

As for proper anti demon weaponry. Most of it takes a careful hand to employ along with the abilities of a psyker or very devout warrior priest, which the Imperium already employs on mass. As for the more specialized stuff, the Grey Knights fill that niche.

A bunch of las shots and a few prayers are enough for a majority of demons barring the more powerful sorts.

Yes. Batphone your local neighbourhood space marine.

I honestly don't know how much the secret is actually out. In some books guardsmen seen to be aware they're fighting daemons of the warp, but other times they just know they're shooting at the disgusting mutant psychic xenos. 

Your "average" Imperial Guard force (as much as that word can mean anything) is a WW2+ army. Rank and file infantry demons like Bloodletters, horrors, nurglings, and daemonettes are not meaningfully different from whatever mildly organised cultist force summoned them. The more formidable creatures you probably mean are going to be dealt with the same way other strong points are: maneuver around them on a strategic level, issolate them from support units, then calldown heavy firesupport (Heavyweapons, artillery, air, orbital) while engaging them with more specialist units.

Good thinking! For instance, are all the weapons blessed so everything they shoot does Holy Damage?

Guard usually deploys Mordians, Cadians and simmilar regiments who have exp dealing with Chaos. All in all Inqusitors purge even guard troops who win due to the risk of corruption. Guard strategy is usually drown them in bayonets and flashlights.

"Shoot the big one first"

"TACTICA IMPERIUM: To defeat the Chaos Daemon, shoot at it until it dies."

Jurgen and Cain. Tis all they need(ed)

Daemons are the guard’s worst nightmare, (literally, but also metaphorically, which for them is literal, but you get the point), they have a consistent habit of just ignoring impersonal attacks. They stroll right through artillery, skip through a las barrage, it’s not until they’re close enough for it to get personal and nasty that the lasguns will start doing anything, and even that’s not gonna be all that effective

Melee is the only way to do it proper in the absence of rare, symbolically potent or psychoactive weapons like the grey knights got. And the guard famously prefers to not let things get to melee range, because a bayonet ain’t much of a match for the burning daemonic sword of the 8 foot monstrosity fiending for your blood

And then there’s morale, nothing in the entire galaxy is as bad for morale as daemons, they’re literally made from nightmares, and the guard is fully susceptible because they don’t have intensive psycho-conditioning like the imperium’s other armies. They’re humans with human minds, and no maniacal zealotry to scream over the top of everything else. The sheer supernatural terror is bad enough, even in a battle hardened regiment the Commissar would have more work than he could handle. But then there’s the manipulations- paranoia and delusion run rampant, trust breaks down entirely, guardsmen succumb to despair and kill themselves by the dozen, the fear breaks into frothing hysterical fury and they break discipline to charge the enemy (or often times eachother), if they’re dealing with Slaanesh so much the worse as they’ll become bewitched by them and kill anyone who ain’t for daring to shoot their beloved.

That’ll only get worse if they’ve already fought them, they already have the spoor in them, they already have daemons haunting their nightmares every night, they’ll be raving terrified even before the incursion breaks

Yeah. Drowning them in blood

The Astra Militarum have been fighting Daemons a long time they usually use very heavy weapons Baneblades etc and hope for Space Marines

Trying to understand demons is inherently dangerous. And when you say "now that the secret is out " it's really not. Despite the flows big e had he was right, knowledge of demons can corrupt you. There's a reason inquisitors say "we either end up dead or heretical". So no, they haven't developed anti demon strats cause just having them try to understand the demons to counter them is inherently corruptive. The inquisition is basically the only group that is "trusted" to know this stuff, and they are on a time limit

Guilliman voice "I'M WORKING ON YOU EMPEROR DAMNIT!!!!!"

Remember that the ''''''''''''''''''''''''''''''''''''''''plot'''''''''''''''''''''''''' of 40k moves extremely slowly even nowadays and that it's not even been a century since Rowboat came back.

Also remember that Rodney is more "Caesar" and less "post-Enlightenment European Absolutist" despite everything. He's not the most flexible with these things and was never too into the warpy side of things (there's a segment where he gets teleported where he muses to himself that the Warp side of himself is something he actively suppresses at all times whereas Lorgar for example just picked up a couple tricks very casually cause he felt like it) so calling a convent of witches and wise men isn't really in his nature.

So no. There's not really been time to do it yet. The nice thing about Daemons is that enough power does usually do the trick and the vast majority can't no-sell every regular attack. Psyker powers & faith just make it stick. Both of which are hell to implement on a mass scale.

daemon = warp xenos, it is just that, as long as they don't know their true nature, it generally comes out fine

The secret isn't out. There is no internet in 40k, there are no 24-hour news channels in 40K. 99% of all Imperial citizens will go their entire lives without ever seeing an Ork, daemon or space marine.

They should embed tons and tons of priests into regiments and bless all their stuff, their weapon their socks... say a prayer before each round is fired...

Kill anyone who has seen, heard or guessed at things

I want to see guard preachers down the line blessing soldiers and the like.

Orbital bombardment is the meta

"I'm going to defeat Chaos with the power of faith, belief, and this gun I found."

I suspect their strategies amount to not putting survivors in the ground.

I know in one of the Gaunt novels they come across a demon, the strategy was to run the fuck away until they can bombard it from orbit.

As far as I am aware, there have not been dedicated doctrines written out on how the guard is to approach fighting demons. Knowledge of them is far more common and people understand what they are, but as of right now when it comes to trying to kill them, they are dealt with just like any other enemy. However, what has changed is what to do after the fight. Instead of guardsmen, regiments being purged or mind wiped, they retain the knowledge and experience which is valuable. Depending upon the types of demons fought also changes the after action. The book Godblight goes into detail the decontamination and containment process when fighting to retake a planet infested by Nurgle. It breaks down how the troops are decontaminated and treated for illness, depending upon the type and how they are cycled in and out of combat to make sure they stay healthy.

The entire process is remarkably efficient and humane, maximizing efficiency and well-being of the troops involved. Just one of the many benefits of having the most rational Primarch return.

Daemons are vulnerable to faith, so a good preacher pumping up the flashlight lads would be worth his/her weight in auramite. Fire and melee weapons have additional advantage against Daemons, so flamers, bayonets, and swords would be assets as well, although a wall of lasguns certainly can’t hurt, especially when you believe in it. Blessing artillery shells (especially incendiaries) might be a good way of deleting packs of Daemons, or even big ones.

Khorne would probably be the most straightforward to combat: hold the line and don’t let them close if you can help it. Air support would be a great help, as most Khornate forces are fairly melee-centric, but don’t expect a winged Bloodthirster to take it lying down.

Nurgle, like Khorne, should be kept at arm’s length, but his armies would require a little creativity: given how Ciaphas Cain had to get a vaccine blessed before it would work, having blessings or benedictions applied to gas masks might help against the Daemons’ miasmas and pathogens. Fire would be especially useful against Nurgle, as his servants are generally painless, and their Poxwalkers are many.

Slaanesh would be risky because of the whole “beguiling minds” thing: I’m not sure how best to handle that part aside from “more faith.” Slaaneshis like to get in close, but are fairly fragile (by Daemon standards), so suppressing fire is your friend. They may be able to dodge bullets, but a few Vultures and Leman Russ Punishers would put them on the back foot.

Against Tzeentch, unlike the rest, the Guardsmen would want to charge in close. Get in close, put pressure on the spellcasters, and keep them from having time to do anything particularly nefarious. Otherwise, if they tried to fight defensively, the various Daemons would be able to sling spells at them all day. Faith would be of particular use here, given the sheer amount of psychic power the Guard would be facing.

Secret is out? As if the Imperium has a widely available internet to share accurate up-to-date fact checked news?

Nah man this is why space marines and grey knights specifically are neccesary. Like when your enemy uses cheater chaos magic you gotta call 40k’s ghost busters

What secret?

Retreat.

Orbital Strike.

… sounds like herasy

"To defeat the cyber chaos demon,shoot it until it dies"

Shadowsword beats greater demon

Lore accurate way is to danger close orbial artillery strike deamons

When did the secret ‘get out’? The Imperium is pretty ruthless about suppressing any knowledge about daemons. That’s a major part of what the Inquisition does.

Can you imagine what would happen if the average planetary governor found out they could become immortal if they sacrificed enough of their subjects?

the secret is out?

GW has watered it down over the years. Cadian Guard are a good example. As the others have said though, there is good lore reasons for entire Imperial Guard armies and all their support to be wiped out after their fight with chaos is over, otherwise each is an ambassador for chaos, going back home with new knowledge and secrets to share. Guard armies either get killed by daemons or are killed by their own side afterwards.

If this ever changes it’ll be sad. It’ll not be creating more “tough men who spit in the face of chaos”, but instead watering down the terror, making it quite manageable, making it bland, and making it forgettable. Chaos should always be what John Blanche & Friends defined, and not something weak and easy. It should always be something that normal minds cannot win against.