This guide is not yet ready, you will surely notice!! I will add a small bit every day, when I have time till i get it complete, so please do not downvote! It helps me to continue the work! When it is finished it will include screenshots and a fully detailed description. :)

So, here it is! A complete guide to Monstrum, a detailed description of the game's items(and not only) that will serve you throughout the game and that will bring you to victory if they are successfully used.

In case you are looking for a specific item, or a more detailed description of a certain item (or you are simply bored to search for the one you are looking for), feel free to skip to the section where the items are sorted by category, and not by letters!

Backpack - Backpacks look like glowy orange life vests. Backpacks are the only items that are not carried, but are instead instantly used when picked up providing the player an additional 2 spaces to carry items. There is no limit to the number of backpacks that can be found in a game.

Bolt Cutters - Bolt cutters (aka Wire Cutters) are one of the most useful items in the game, if the player is planning the helicopter escape route. Not only can they cut the restraint cables holding the heli to the ship, but they can also open the containers surrounding the heli and reveal the fuel pump (they used to reveal a bunch of fuel canisters but as of update of May 15, 2015 this is no longer possible), that pumps fuel inside the helicopter, thus enabling the player to complete 2 of the heli tasks. Like most items in the game, the Bolt Cutters don't stack.

Chain Spool - The Chain Spool or simply Chain, is required to fix the liferaft crane, allowing you to lift it off once it has been fixed. It is therefore the one of the three items necessary for the raft escape

Duct Tape - The Duct Tape is one of the items necessary for the liferaft escape, since it patches up the holes and allows the player to pump it up. Doing so will use up all of the tape. However, it has another use. It can completely block the security cameras at the upper decks (aka Crew) and allow the player to explore to his/her heart's content (what a lovely expression to use while being chased by a monster). Although the Duct Tape does not stack, it is a small item.

Egg Timer - The Egg Timer is a small unstackable object that serves the purpose of making a noisy distraction. Although it is difficult to spot, if used it can make an extremely loud ringing that can be heard all over the ship (SERIOUSLY!!!).

Fire Extinguisher - The Fire Extinguisher can be found hanging from the walls in any corridor of the ship, but is most commonly found near the steam pumps. The Fire Extinguisher can shoot a limited amount of its contents on the monster's face, stunning it for a few precious seconds in a chase and buying you time to escape. Remember it the next time you see the Brute! It does not stack and it makes a small thud when it hits the ground.

Flare Gun - The Flare Gun is an item with multiple uses. It can do 3 things, make a loud noise that serves as a distraction, hit the monster and stun it for a couple of seconds (for FPS players only) and it also lights up an area, however it is not recommended for the last use due to the limited amount of ammo and the noise.

Flashlight - The Flashlight (aka Torch) is generally considered to be one of the most useful items in the game, since it illuminates brightly dark areas and can also be switched off in case of a chase. Along with the Fuse, these 2 items are always found in the opening room, where the player starts. Unfortunately, the flashlight battery sometimes dies, meaning that the player will either have to search for a new flashlight (unlike common belief, several can be found in a game) or use a different light source. Flashlights unfortunately do not stack.

Fuel Canister - The Fuel Canister (aka Gasoline) is a relatively common item, that is used for the helicopter escape. With two canisters, the player can completely fill the fuel pump and open it to fill the heli. Fuel is also ignitable if thrown on the ground and can be ignited via lighter or flare gun. Fuel does not stack.

Fuse - The Fuse and the Flashlight are the items with which the player begins his search inside the ship. Fuses stack endlessly and are small items that can be found in drawers. The fuse's only use is to empower a fusebox. There are 3 types of Fuseboxes that do 3 different tasks:
1) Open the lights of a whole deck (lower or upper) and allow the player to explore without the need for a light source
2) Empower the submarine room, filling it with light and allowing the player to use the control panel and escape
3) Engine room fuseboxes power up the telephones in the engine room and also light it up.

Glowstick - The Glowstick, updated as of May 15, 2015 to a new look, is an alternative light source, which can be used by the player to illuminate dark areas.Unlike the flashlight, it is "nearly" impossible for a glowstick's light to die out. Glowsticks can stack up to 3 and are small items that can be found in drawers. Glowstick light is not very bright, but a practical tip is that it tends to shimmer on a wall with a Hunter's egg sack, allowing you to detect it more easily.

Helicopter Keys - The Helicopter Keys (aka helies), as the name suggests, are the keys required to unlock and open the helicopter door and are the second necessary item for the heli escape. After their use they are destroyed and they have absolutely no other use in the game. The Helicopter Keys are small unstackable items that can be found in drawers.

Lighter - The Lighter is another alternative light source that is not as bright but is very trustworthy. The lighter makes use of a small flame to light up the path ahead. The lighter is unique in the sense that it can also light up quantities of fuel and ignite them. The lighter can also be found in drawers and is unfortunately unstackable.

Pump - The Pump or Air Pump is necessary for the liferaft escape. It is required to blow the liferaft, but can only be used after the consumption of a duct tape, lest the air escapes through the holes. Pumps are rare and unstackable.

Smashables - These include bottles, mugs and glasses. The smashables can stack up to 3 and are used for a noisy distraction of breaking glass. They are the most common items in the game. They can be found in drawers, making them a small item.

Submarine Battery - The sub batteries (aka Subblings) are put in the side of the submersible and power up its engine, making them a necessary item for the submarine escape. They do not stack and are known to be found mostly in common rooms (Search Well!!)

Submarine Headlight - The sub headlight (aka Big Brother, because it is big!) is the missing headlight on the submersible. In order for the player to escape with the sub, it is necessary that the player puts it on the submarine. It is unstackable and its only use is for the sub escape.

Walkie Talkies - The Walkie Talkies used to be an item that would make a false talking sound in a different area of the ship in order to create a distraction. It was unstackable and rare, but as of the update, they have been removed from the game

Welding Kit - The Welding kit is an item also required to escape with the submarine, since it helps you to weld and fix the broken panel on the submarine's side. Apart from this particularly important use, the welding kit can also shed some light on the dark areas of the ship with its little flame. The welding kit also has a key symbol, meaning that it can lock a couple of double-doored rooms forever, by welding the lock closed.

The Fiend
The fiend is the scariest and personally the worst monster (Fiendzoned is still a bit too far for me). The Fiend uses many techniques that can surprise the unwary player. One technique of surprise is that the fiend simply knows approximately where you are! BEWARE! If you know you have the Fiend, something that can be found out by simply watching the lights, if the Fiend is near they will flicker, watch out from extreme flickering or the flashlight dying out. This will probably mean that you have to hide to avoid contact with the fiend. Know that if he is chasing you he can launch a ranged attack at you freezing you for a few seconds just like you can do to him with the fire extinguisher, only you can freeze him from close up instead of from range.
He can easily find you and, due to a supernatural floating abilty of his, you can hear a calm hum if he is near you. It is extremely important to know that he is smart and that he wll probably check all of the hiding places if he saw you enter a room.
WARNING!: DO NOT ATTEMPT TO APPROACH A STAIRWELL IF YOU KNOW THAT THE FIEND MIGHT BE NEAR IT! HE IS PROBABLY THE DEADLIEST ON A STAIRWEL!
Why does this happen? Because the Fiend's last and deadliest weapon is his ability to grasp you if you are a short distance from him.He cannot do this from a long distance but in a short one such as the stairs.... INSTAKILL!!!

The Hunter

As mentioned above, there are three ways to escape from the ship. The first one is the helicopter or heli, the second one is the submarine or submersible or simply sub, and the final one is the liferaft or raft. Here are the items and techniques you will require to escape, so that no one has to write your tombstone!!

The Helicopter
The helicopter is the first escape route. It used to require you to find only 2 items, the Bolt Cutters and the Helicopter Keys, with the bolt cutters being the most precious, since they allowed you to open the cargo containers near the helicopter and therefore find the fuel pump and Fuel Canisters. However, as of the May 15,2015 update, fuel canisters can no longer be found in the cargo containers so the player has to track 2 fuel canisters by him/herself.
After the May 15,2015 update the helicopter has been made tougher in another sense. Steam pumps have been added near the heli and the player has to be careful so as not to get thrown back by them in a chase. Despite that, if they are placed on the correct side, the player might be able to delay the monster for a short while before it arrives at the geli deck.
The helicopter is situated outside the ship. On the main deck outside, climb one of the 4 stairwells to reach the helicopter bridge. Watch out for the steam! It cannot kill you, but it can blast you backwards!
From the one side of the helicopter, the door can be found, which can be opened by the helicopter keys. It is the only use of this rare and small item. The bolt cutters complete the task of cutting the 4 restraint cables of the helicopter and opening the containers, revealing the fuel pump to the player, so it completes 2 tasks.Attention! The fuel pump needs to be dragged as though it is a cart to complete the task of attaching it to the helicopter, and then the player needs to pick up the pipe that is hanging from the helicopter and put it in the suitable hole. The fuel completes a separate task, filling in the fuel pump.
Get ready before you open the fuel pump, because then you will be asked to protect it by causing a massive distraction for the monster. The fuel pump is extremely noisy and doing so is tough. The task requires you to bring the monster away from the pump, so the monster does not destroy the pump. If it does, all the job done on the heli willl be wasted, because it cannot be redone.
There are 2 possible ways to complete the task. The first and highly suggested, is to stand by the fuel pump. Due to the added steam vents near the helicopter, you need only watch the one side of the stairwell. Then make sure it starts to CHASE you, doing so will trigger the monster's music. Then feel free to run toward the furthest side of the ship (crew or cargo). and escape. Once the task is complete simply run back to the heli. Entering it will trigger the winning cinematic.
There is another way, that involves the cameras spotting you. I have tried to distract the monster through the use of flare-gun and smashables. It did not work, since the monster went for the fuel pump instead for the sounds. But the camera distraction has certainly worked and allowed me to complete the task.

The Submarine
The submarine is located on the cargo deck (lower deck) 1, the lowest deck in the ship. I can say that it too requires a waiting time (2 minutes) after the completion of the items.
You need to place the batteries, the headlights and fix the panel with the welding kit, as well as fuse-up the room. The fusebox and control panel for the submarine are situated on top of a ladder in the sub room, so you need to place the fuse there. All the other items need to be used or placed on the submarine itself.
The tough concept of the sub is the fact that it requires.a two minute waiting time after pressing the release button on the sub's control panel. Doing so will start to lower the submarine, but trigger a massive alarm in the ship that will literally rattle the walls. The doors to the room then lock, preventing you from exiting and causing you to get trapped in a room with a mean monster once it smashes up your doors. Use the carts to block the doors, there are multiple and it is plausible to do so. For the additional door you can set up a fire and block the monster.It cannot get past the carts I think and doing so is a good idea.
The second way, that is untested is suggested to the risky only. once the monster blasts in the sub room. Run around and escape through the door he opened hide somewhere, then cause a distraction and run back to the sub to win the game.

The Life Raft
The liferaft requires three items to be completed. These items include the Chain Spool, the Pump and of course the Duct Tape. The Chain is simply placed on the crane, near the liferaft on the outer part of deck 7. The duct tape is used to mend the holes of the raft and after that, the player can pump up the raft and make it ready to sail.
Then, you are asked to grab the raft's top with the crane's grappling hook. This used to be tough and bugged, but it has been about 2 months since I managed to escape and I supposed it has become easier and unbugged. The only problem with that is that you have to use the crane's controller which is not simple to use and is also noisy. But it is very plausible to manage it.
Once the crane is very close to the raft, you have to attach it to the raft manually, the same way you attach the fuel pump to the helicopter. Then go back to the controller, lift it off the ground and lower it to the sea. Then you have to access it by going outside deck 5. Once you have approached it, the game will end and you will have won. Congrats!!!
For me, the liferaft is the easiest escape route.

The game begins with 6 slots, 2 of which will be occupied by the flashlight and the fuse and after a while another will be probably occupied by the glowsticks. This basically means, that if you find 3 more useful items you will have no more space for any of the numerous items in the game (see section 2-Items in Alphabetical Order). So what can you do if it is necessary to take an item (for example a pump if you have duct tape and a chain spool)? Here is some useful advice to aid you in such a circumstance.

• When your inventory is full or reaching full, consider using some of the items by placing them on the correct vehicles (liferaft, sub, heli). Items such as the helicopter keys, the chain, the pump and the sub batteries and headlights can be used to save up space for other useful objects. (fuses can also be used up and placed in fuseboxes, but it is not highly recommended due to the fact that they have many uses, unlike usable vehicle parts)
  
• Search for a Backpack that is especially useful in order to increase your inventory by two slots and aid you in carrying more items. The more backpacks you take, the more additional space you have.
  
• Avoid picking up Smashables for 3 reasons. First of all they are not the best distraction in the game. Secondly, they are so bloody common that you can find one or two in every room in the game if you ever need a quick small distraction. Finally, unlike other items, they cannot be dropped without making a sound (even if you drop them in the sea), so not taking them in the first place, saves you from having to run.
  
• Judge your patience well. This may sound like a stupid piece of advice, but it is indeed extremely useful. What I basically mean is to judge how much you will be scared when you encounter a monster and whether you will be patient enough to blast it with the fire extinguisher or with the flare gun. If not, avoid taking any of these two items and especially the fire extinguisher, since it also makes a sound when dropped.
  
• Do not hold on to your dead Flashlight. For it serves absolutely no use and doing so simply uses up space that could be otherwise used for something useful. Throw it away, since it makes absolutely no sound and will give a whole empty slot.

Distractions play a very important role in the game. They are necessary to access barricaded rooms and can also fool the monster buying you precious time. However do not produce distraction sound where you are. That is called trolling and will probably get you killed!

Quick Facts

• A distraction is a loud sound, that in the quietness of the ship can be easily heard all over it. They can be used as the name suggests to distract the monster away from you or to a certain place you want it to be (e.g Barricaded Room)
  
• Distractions can be produced in many ways one of which is items. The items that produce distraction sounds include the Flare Gun, the Smashables, the Egg Timer and the Radio.
  
• Other sources of distraction are cameras (you can let a camera spot you on purpose) and the engine room phone box that will lure a monster to a room of the engines that you want it to be (steam room for example).
  
• The Walkie-Talkies used to be a distraction producing item. As of an update however it has been removed from the game.
  
• Sounds are also produced by loud footsteps and other sources, however these can only bring the monster if it is already close enough to hear you and do not as such count as distractions. They also bring the monster to you if it is very close and such an action is generally not greeted with enthusiasm as distractions are.

Distraction Tips and Strategies
Distractions are an extremely important part of the game.

I have found all of the achievements and so here they are. Beware of spoilers ladies and gentlemen!

A Sequence of Unfortunate Events - Get killed by a monster after falling on a pit trap.
Aim Away from the Face - Stun a monster with the Fire Extinguisher
Bait n' Switch - Trigger a Hunter Trap with an item.
Charlie's Devils - Get slain by all the monsters
Child of the 90's - Have 6 Glowsticks at once
Fiendzoned - Win the game against the Fiend
Got To It - Escape via the Helicopter
I don't know what I expected - Hit the Brute with a flare.
I love it When a Plan Comes Together - Gain access to a barricaded room
Let there be Light - Power 2 additional areas in a single playthrough
Now Think About What you have done - Lock a monster in a power-locked room
Steam Won't Connect - Close the main Steam Valve
The Hunt for Me is Over - Escape via the submarine
The Most Dangerous Game - Win the game against the Hunter
Well Done! - Win the game against the Brute
Women and Children First - Escape via the liferaft

The easiest and quickest achieviement is probalbly Charlie's Devils. You do not even have to try, just die a time by each monster. It will probably happen anyway during your Monstrum career. "Easy" achievements include the non secret ones. Just go and win the game! (not easy btw)
See the Section stopping the monsters to see the tips on spraying a monster with the Fire Extinguisher which is an achievement and a very easy one.