Skulls were bugged for many people and the 10th skull crashed the game, but I assume a couple years on that's now fixed? In case you don't want to risk losing progress if you're trying to do multiple achievements in a single run, I would leave the skulls for it's own run unless you know it's not an issue.
The 12 minute speedrun achievement is very self explanatory, recommend recruit, just hold W as much as possible and take characters with speed like slayer or dashes like HM, aim to enter the final boss area by 7:15 at the most - I got there twice at this time and once finished with 3 seconds left and another time with 30 seconds left (Olesya talks a few seconds before bubbling).

(1/3) - At the dropdown area just before the potion room, you'll see a slab with a symbol (Triangle, Circle, Arrow), step onto the same symbol to unlock two gates on the right side of the potion room and unlock the first tome. There is a ladder also into next area at the back.
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(2/3) - In the bookcase room where you can platform onto a bunch of them, there is a tome on top of these, second in on the left side. You can climb up from centre of the room or crouch jump onto banister before jumping on first one in on the left.
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(3/3) - After climbing the giant "Lustria?" tree and entering the room with ropes hanging down, instead find some climbable vines in the same area, climb these, follow a path and you will find the tome on a dead dwarf.

(1/2) - In the "destroy the staffs" room, go up to the platform that has the chandelier sticking out from it, look down and you'll just barely make out a transparent floor tile - jump down to this and you will find the grimoire, also will be 2/4 on Liar Ahead. (works currently jumping off the table)

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(2/2) - When you enter the transpositional portal (swirly painting), halfway around the area you teleported to will be a gargoyle head you can pick up. Turn around and backtrack to the entrance where you'll see a gargoyle with a head missing. This will open up the flame door and through the cave is the final grim and book.

In the potion room, you can keep mixing potions until you've made all 3 - you can make enough potions + healing draughts for everyone.
Potion key:

PYB = heal pot | PBY = concentration

YBP = heal pot | YPB = strength

BPY = heal pot | BYP = speed

Credit to valaaan on Reddit

In the first drop down where the gravity has been reversed (floating blue particles and wind) will be a symbol on the wall (Triangle, Circle, Arrow) - this code is used to unlock the first tome on the right of the potion room, and when you reach the invisible bridge part you just have to walk out from the right stone slab with a matching symbol to earlier.

(FIRST) Left of the potions table in the potion room, you'll have to wander around till it triggers.
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(SECOND) In the "destroy the staffs" room, go up to the platform that has the chandelier sticking out from it, look down and you'll just barely make out a transparent floor tile - jump down to this and it's also for Grim 1.

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(THIRD) In the Lever room, go to the right, there is a room behind the right lever on ground level.
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(FOURTH) Go to the top back of the lever room, to the right of the golden door and down the fallen broken ladder.

In the Lever room, there is a picture in the corridor at the back on ground level, telling you to pull the levers in the order of 1532 - once you have pulled levers 1,5,3,2 - go to the hidden room on the right when you entered, and pull the lever, and back to the corridor with the picture where shelving with barrels have moved to reveal a secret passage which is a replicated are from the first map that this level is paying homage to - there is also a skull here which may have been eluding you.

Alternate Codes:
5123 - Found in a secret room behind some curtains in a feast hall, on a green book next to a dead guy in a chair. Spawns healing and potions on the tables.

4361 - In the secret lever room, there are numbered barrels, left to right only counting barrels from 1-5, you will get this code. Will spawn a Rat Ogre.

3421 - The star clusters that the wolf in the telescope runs to in order. Supposedly opens the gates at the end event, anything associated with those is a bit buggy and can crash your game, at the moment.

In the "Lustria?" foresty map where you climb a giant tree, go instead up a cave on the right side when you first face the tree, at the top you will find an interactable gargoyle that will give you this achievement - it disables fall damage for the rest of the map.

Skull 1 - First building you walk through after cutscene
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Skull 2 - On the far left side, before you do the first "dropdown" into the potion room
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Skull 3 - Far back left of the potion room
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Skull 4 - Right under you in the sewage where you activate the invisible bridge
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Skull 5 - Next to the blue fire staircase while Sofia talks
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Skull 6 - In the "break the wards" room, go as high up as you can and into a small corridor at the back
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Skull 7 - After the ward room, there is a sewage room on the right with a dropdown and skull
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Skull 8 - Where you climb the giant tree in "Lustria?", on the far left
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Skull 9 - After the "1532"/Return Visit puzzle, follow the secret path and there will be a skull before some red curtains and a skeleton with a key
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Skull 10 - In the narrow corridor before you are in the Olesya/valve room, in the corner of barrels is a skull
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Credit to Valaaan on Reddit for helping me find all these

Check out this really cool art someone made of a "Map" of the Tower of Treachery!
https://www.reddit.com/r/Vermintide/comments/1275amg/map_of_tower_of_treachery/

u/Kim_sben