Are you using Anoint in Blood, Purge the Unclean and Ambuscade talents? Flamer with unarmoured and infested as perks and Blaze away and Overpressure as blessings. Try getting a damage stimm aswell. Use stimm, throw grenades and prep flamer for continuous fire and use Fury of the faithul should give you enough damage.
I can’t remember what most of the perks are called unfortunately. I’m dyslexic and my memory for words is extremely poor. I can’t even remember the names of the difficulties (I’d much prefer a number system or easy/medium/hard/nightmare etc). But I did just try your suggestion of malice archivum sychoratz. Breezed through it in no time and even ran off by myself at the end and attracted 2 of the larger horse waves but thwre just wasn’t enough. Killed all of them and my highest go is now 19 instead of 15.
So I don’t think malice has enough enemies. It’s a total ghost town on those lower difficulties.
I’m starting to think this penance was designed when there were actually enemies in the game besides auric damnation.
Heresy is difficulty 4. High intensity is a must.
Some events even there will not have the required density.
All good points thanks. Although the timer only starting on the first kill doesn’t matter too much if the stun effect only lasts a few seconds… Unless it works that if I stun a crowd and they become unstunned and then I kill them and it counts… If that were the case then I’d be getting a much higher count. So if they have to be still stunned whilst being killed then really I have less than 10 seconds to kill them. Beyond ridiculous.
You’re right it is totally stupid. What an absolutely idiotic penance. It’s making me so angry.
I’ve now had to give up on my 4th failed night. No fun at all. So that’s 4 days of zero fun trying to do this POS bit of lame design.
I’m starting to think that it was made for when there were actual hordes in lower difficulties.
I’d be curious if anyone has actually done it recently.
I don’t even want to play the game anymore. It’s the first time I’ve felt that since discovering DT. All because of this game-ruining penance.
Lol well there’s immediate proof of my dyslexic word memory 
I’ll try that difficulty if it ever shows up on my lobby. It didn’t show up once in 4 hours of playing tonight.
Iirc the stun nade lasts for 6s total. It only applies to the enemies caught in the initial blast, but keeps reapplying stun to them until the 6s have passed (also stacks stagger talents & blessings btw, capping stuff like Pinning Fire instantly etc.).
Anyway since there’s a significant delay before the nade goes off the idea is to for pop the nades to say, areas A, B and C to cover as many enemies as you can, and immediately after throwing the nade to C switch to ranged and start cleaning the zones in the same order. So start with A before the stun wears off, then B and finally C. You should have a big zone of enemies to do this with and idk 8+s of stuns total to work with.
Also I only saw your difficulty comment now. Absolutely do this on T5+, like Damnation Auric High Intensity minimum. Most all penances are best done there since they have the highest spawn rates. It’s the only fun difficulty to play on anyway. 
But… regarding penances in general, I definitely think you’re on the right track “giving up” on it. I couldn’t even imagine chasing them on purpose. Fun first, penance second. Use the chance to try out builds you haven’t before and keep trying ofc, but don’t fixate. Switch characters and builds - for other penances if you like - when you feel like it to keep things fun and fresh!
Okay. Try modifying your build specifically for this achievement. Try to take an automatic and rapid-fire weapon with more or less normal damage, something like ColumnusV assault rifle and things like that, to squeeze fire into the upper body of enemies and have a fast reload. Next, get a melee weapon that will allow you to kill almost all enemies with one hit, something like a Rashad axe, this is especially in case you run out of the 2nd magazine and need to finish off a couple of enemies. And of course, in these weapons put +% damage on trash mobs and all sorts of blessings to increase damage.
As for the bolter, there should have used a blessing that increases damage from the number of stunned enemies.
You can also make a NORMAL build on melee weapons like chain axe (Rashad is better, of course, but I liked this chain axe with its animations and splash damage, for the axe Rashad build will need to change a bit.). The idea is to build up buffs by killing 5 enemies, then use a grenade and mow down the entire mob. Hit ONLY with light attacks, against mutants and other single fat targets you can use a powerful attack, usually 1 hit and the enemy is dead. And yes, you need to have two health sticks, at the beginning get a few hits to lower your health and then have fun. Here’s a link to the build.
And, of course, use the red syringe on damage. I often notice that often players don’t even use them, even though in Vermintide 2 cans were drunk by everyone.
I’d also like to say. If you have this one achievement left in the class and plan to jump to other classes, then yes, you are in trouble.
However, if you plan on just playing this class next. Just make a good build through stun grenades, play and have fun, on gold missions we’ll easily complete this achievement sooner or later and you won’t notice it yourself. The main thing, do not make a fanatic shooter. Maximum for achievements. But for a normal game, a fanatic shooter is a terrible problem.
And yes, I read some of the comments and tried testing the bolter and weapons in the current patch on the fanatic. It’s not very cool. I will say that I think this achievement will be easier to complete with melee weapons. Try to pick missions where you have huge hordes of enemies running at you at the end.
Thanks for the comments, all good points.
Did you do the penance recently though? Because finding hordes as plentiful as to supply 1 player with 40 kills in 10 seconds is actually incredibly difficult. Apparently horses used to be way denser way back before cross play, presumably what this BS penance was designed for.
Even on auric damnation (which is the only difficulty where there are enough enemies, but then they always throw in 10 crushers or maulers as well so that nerfs any attempts at 40 kills).
Stun a pox walker horde, take out flamer. Basically stun any horde and use a flamer.
Penance is bit dependent on the teammates. You can use a flamer and stun a big group in basically any event, if they aren’t cut down or get separated by teammates running around like madmen as usual.
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When you have to kill a relatively high number of enemies within a limited time, you are only sabotaging yourself, if you do not do it in a mission with the highest enemy density available.
If you are not a good enough player for that difficulty, just do something else until you are. You are not forced to complete this penance right now (or at all). -
Use the flamer. Yes, flamers do not instant kill anything, but when it comes to removing large groups of enemies, a flamer is great. 10 seconds is plenty of time to kill enemies with a flamer.
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This penance was not very difficult before and it got easier with the recent update:
- Get a friend. Start a private damnation high int mission and finish the penance on the first horde that spawns.
Takes about 5 minutes.
Lol are you okay?
Events will have a more fixed wave layout of enemies if you don’t want Crushers to ruin the attempt with trying on a horde alarm.
Well with the people on this forum, you never know. 
ah jeez rick
Everything n everyone gotta be taken for a joke at first glance (except obvious sht like safety). Less stress that way.
Well, with the build I gave, I think fat targets that will be in stun won’t have time to hinder you. Also yes, the flamethrower option. Just set the targets on fire, and then throw a grenade, and keep burning.
As for the achievement. I did it almost immediately before the game even started, once I got my hands on the bolter. Judging by the date on the steam 19 nov 2022.
Ooh! That’s fantastic! I may have to do some stun builds now!
But yes, @MadMartigan they are right that flamers can work too. Since the tracking starts from the first kill and flamers do DoT, you can first puff a few stacks of burn everywhere or into the first group in particular → pop the 3 nades → start burning them for real. If you time it right, they’ll be heavily damaged and start dying from the DoT just as the first nade goes off, and then you just finish off the rest with the flamer.
So again, either Man-Stopper & Scattershot Kantrael or the flamer definitely have the best AoE kill potential of all the weapons you have available. I would not even attempt something like this with the infantry autoguns as some have suggested. They are not AoE weapons, and especially when you have to clear a whopping 40 enemies in 10 seconds.
Shotguns? Good AoE weapons? The thing is, when some Columbus V may not do AoE damage, but it hits with pinpoint accuracy and 1-3 bullets to the head or body and the pox is dead. Reloading is fast. And the damage is good. Whereas shotguns have AOE damage, but the problem is that the AOE damage is spread out between targets. Let’s say you have 16 pox running at you. You have 8 rounds. Each shot you fire is divided among 16 mobs. How fast will you run out of ammo? And will you kill at least 16 regular pox? And this sh*t’s reloading is even worse than the Bolter is right now. Yes, you can interrupt it, but how much time will be wasted already. While taking the automatic columnus and the right blessings will just elementally put down the entire crowd with 1-2 rounds without any nonsense and activation skils. And even if you use skils, mmmm.
P.S. And one more thing about shotguns. Many times I’ve seen how some, how to call them, mmm, shooters… Just choked in the horde with their shotgun. And yeah, the only thing that saved them was invisibility. In short, they were of no value to the team, both in terms of damage and support. And yes, if the Veteran with a shotgun can still be somehow imagined, then a fanatic with this weapon will look just stupid and useless on gold missions. And also take into account the fact that you need to survive there, but to give a volley, kill 25 pieces and die, well, so-so.
And yes, from the looks of it, a flamethrower would be the best option for this achievement. Blowing on a few stacks. Dropped the Grenade and blew the final one.