I am writing this guide because I could not find one in English for Strange Brigade. As I like the challenge of getting to 100% in a game and have referred to similar guides for other games, I thought I might as well put one together to (hopefully) help someone else.
Spoilers can ruin the fun of discovering things, so any hints that could spoil secrets are hidden like this for anyone who wants to use this guide while still figuring things out on their own as much as possible. For a lot of achievements, the descriptions already contain hints and in some cases even the steps how to get the achievement - the developers seem to have wanted everyone to be able to get those easily.

If you are hunting for achievements, you need the three "Thrice Damned" DLCs and someone for the multiplayer achievements. The Strange Brigade can easily be completed in about 40 hours, or less if you would refer to a guide for all collectibles. Extreme difficulty can be a challenge when playing solo but would become much easier playing with a good team.

Disclaimer: This is my first guide on Steam. Please bear with me if the style is simple. Any advice and suggestions are appreciated. Images used in this guide are based on screenshots taken in the game or Steam's own achievement page.

A central part of the game besides the puzzles and combat is keeping an eye out for various items scattered throughout the levels. Strange Brigade has five kinds of collectible items with corresponding achievements:

• Cats
  
• Canopic jars
  
• Diaries
  
• Relics (main game levels)
  
• Figures (DLC levels)

Canopic jars and diaries disappear from a player's view once they get the item, so keep this in mind if you are playing with friends and want to point something out to them.
Progress for a level is easy to track in the summary the game provides, so you can check what you may have missed and which levels to play again (in this example one missing diary, with four canopic jars, six relics and cats already collected):


There is a video guide that shows the location of every collectible item, helpful for that pesky last item in a level that you missed.

Statues of cats are scattered throughout the levels. If you destroy them, a ghostly shape remains. Finding all of them opens a door at the end of the level which leads to a golden cat. These items can be collected every time you play a level.
Listen to cat sounds while you go through the level. Whenever you hear one, a statue will become visible somewhere around you. The blue color makes them easy to find, though they may be partially hidden or further away sometimes. The final cat in each level is located near the door at the end, but it only appears if you have collected all the other statues.

Hooked on a Feline
Collect all the Cat Idols


Every level contains a couple of diaries that tell the story of the game from the point of view of different characters.
A lot of them are found near camp sites or behind puzzle doors, but not all of them.

Purloined Letters
Complete the Main Diary Collection


Every level contains this kind of pottery, and you get achievements for breaking them all. Besides that, they serve no immediate purpose.
The jars have a distinct blue color which helps in spotting them. Usually far off and easier to get if you have a gun with good accuracy, but not always found at a distance.

When is a door not a door?
Collect all the Canopic Jars


Found only in the main game, they are listed in the journal in sets. Getting a full set allows you to unlock an amulet. Relics are often found behind puzzle doors.
Levels have one relic that is easy to miss because it is carried by a "thief" who will run away when the players get near. You have some distance to chase them before they disappear. Shooting them drops the relic to the ground.

It's not theft if you put it in a museum
Find a Relic


To preserve the past is to save the future
Complete the Main Relic Collection


Master of the Mystical
Unlock/Find every kind of Amulet and Gem
Unlocking every amulet means finding all relics. If you complete the game without coming across every kind of gem, you can farm them from chests in any easy level until you do. There are other guides with hints for doing that because the gems can be powerful additions to your guns and are worth gathering even if you do not collect achievements.

Every DLC level has its own set of achievements for collectibles, and figures take the place of relics, but otherwise the collectibles are quite similar.

Cat got your tongue?
Collect the golden Cat Idol from the jungle


Well-Read!
Find all of the Diaries in the jungle


A smashing good time!
Destroy all Canopic Jars hidden in the jungle


Pirates' Plunder!
Dig up all of the figures lost to the jungle


The cat that got the cream....
Collect the Cat Idol from the cave


Knowledge is power!
Find all of the Diaries in the cave


Conopic chaos complete!
Destroy all Canopic Jars hidden in the cave


Treasures of the Earth!
Unearth all of the figures lost to the cave


Combing the Cat-a-combs
Collect the Cat Idol from beyond the Pyramids


...and read all over!
Find all Diaries near the Pyramid


Don't go breaking my jar!
Destroy all Canopic Jars hidden at the Pyramid


Ancient Action-Figures!
Find all of the figures lost in Egypt
Nobody seems to have lost any figures in the rest of Egypt outside of the pyramid grounds.

By the time you complete the main story, you should have collected several of these achievements automatically. You will find hints how to get the rare ones below.

Stiff Upper Lip
Kill 20 enemies while on low health


The trick is not minding that it hurts
Spend a total of 20 minute at the low health level


Not Out
Kill 501 Enemies


Capital shot!
Kill 3 enemies with a single shot
If you did not get this automatically while playing the main story, the simplest way is to start a level at low difficulty and grab your best shotgun. Some limited weapons purchased during levels also work well. You can even do this easily at high difficulty by weakening enemy health with your pistol before switching to a gun with good spread.

These Are My Fighting Trousers
Melee kill 5 enemies in 30 seconds
Depending on your style, you either got this already or will need to do it on purpose. Remember to stomp on the undead after you knock them down.

Queensberry Rules, ok?
Flatten 100 enemies with a melee attack


Hydra's Teeth
Kill 100 skeletons


What have you got against pottery?
Destroy 100 pots


Nobody Likes a Show-Off
Chain 3 Amulet Attacks in a row
This may not happen while playing the main story, but it is quite easy in horde mode with other players killing enemies once things get busy. I got it while trying out the lightning amulet and playing a character with increased magical powers the first time I played horde mode with a friend.

For Medicinal Purposes
Drink 6 health potions within a level without dying or restarting
Quite easy to do on your own because most levels contain enough potions. Just use them as soon as you find the next one. Explosives come in handy if you need an excuse for a drink.

This Is Becoming A Little Awkward
Exit a Sarcophagus 5 times without restarting
Similar to the previous achievement, you can find some enemy and get yourself killed whenever you locate the next potion. Just do not have a drink for other purposes.

The Pharoah's Curse
Die 50 times

A few achievements require a multiplayer game. If you are in a hurry or just prefer playing on your own, you can do them in a single level with one other player helping you out.

Now That's Just Not Cricket
Kill a teammate with any kind of trap


Kudos if you did not get this while playing the entire campaign in coop mode - now you have an excuse for doing it on purpose. :-> Oh, the things people do for achievements...

I'm stuck with a valuable friend
Collect double the Gold of each of your teammates


Everybody needs a hobby
Rescue a teammate from a sarcophagus 10 times

These achievements reflect the progress in the game. Most of them cannot be missed and contain the level name in their description. The list is ordered, that way you can tell if a level has more than one achievement. I have chosen to hide all relevant descriptions even though the developers only hid those of most main game achievements, because I want to avoid spoiling anything unnecessarily.

Wonderful Things
Open the Sarcophagus Room in Harbin's Dig Site


Bully For You!
Defeat The Champion in Hidden Valley


Save Our Souls
Destroy a Soul Cage


The Mote out of thine eye
Complete the Beam Puzzle in Tunnels of Wadjet


Get down, Shep!
Defeat the Guardian in The Towering Temple


Hammer of the Gods
Kill the Hammer Champion in Cursed Village


Dangers Untold
Solve the Maze and open the door in Cutthroat Caverns
You can miss this one if you step past the last checkpoint into the final battle.

A secret door to another world?
Open the Hidden Relic Room in Forsaken City
You can miss this one if you accidentally step past a checkpoint without solving the puzzles along the way.

Eye say!
Activate the Death Ray in the Great Pyramid level


Smile, you Son of a B…
Shoot Seteki from Ammit's Jaws


For King and Country
Banish Ammit and defeat Seteki without delivering the Coup de Grace
AKA, do not shoot Seteki from Ammit's jaws. The two Ammit & Seteki achievements are mutually exclusive, so you need to play the level twice to get both. That is probably the reason why the developers gave a detailed description instead of hiding these achievements.

By George, I think they've got it!
Collect the first half of the Shield of Bes


The heart of the mountain!
Collect the second half of the shield of Bes


Is that the Bes you can do?
Reassemble the Shield of Bes


And stay down!
Defeat the evil that lurks in the Pyramid


7 league boots
Travel for a little over 24 miles
While not exactly part of the story, your mileage will pass this mark automatically when you play the main campaign, typically in the level "Cursed Village" if you just follow the storyline.

Tinker, Tailor, Soldier, Savant
Solve every kind of puzzle door across the game
This should pop during the level "Forsaken City". What would be the purpose of playing a puzzle game if you do not open every puzzle door you come across? ;-)

Pay the Piper
Open a Gold Door in Horde
Any door that requires gold to open it.

Evasive Action
Get to Horde Wave 4 without taking any damage
Take your time, do not let enemies get too near. You can always dodge and run halfway around the arena before attacking again. Pick places with one main approach and a back way to get out if it gets too crowded.

Back From the Mouth of Hell
Complete the First Round in Horde


There's no 'I' in Team…but there is a 'me'
Complete a level with each of the first four Brigade members


Minted
Collect 10000 Gold from Chests
Well... not really a spoiler... but if you have been playing multiplayer only and did not get this, just do a level in single player.

Master of the Armoury
Unlock all the weapons
You get lots of cash playing the campaign in single player, so this is pretty easy. I would recommend unlocking the expensive weapons early because they are useful when playing solo (see extreme difficulty, below). Most standard gear becomes useless if you have DLCs.

One Simply Doesn't Rush These Things
Annoy the Narrator
Do not move for a minute or so until the narrator is done with his dialogue, then move.

Snap Decision
Enter photo mode


Portrait of an adventurer
Unhide the player and hide UI in a picture. Point camera at the player


Definition of courage
Complete the game on Extreme difficulty


There is a guide with advice on beating "Forsaken City". A lot of the advice in that guide is useful in other levels as well.
https://steamproxy.net/sharedfiles/filedetails/?id=1803901822
While the levels can be done solo, some spots become quite challenging if you lack diversity in guns and cannot handle single targets at range as well as bosses that charge you.
When I did the levels, I went with the Audley Burst Rifle for its versatility along with both decoy grenade and the "Call of Seteki" amulet. Frank as a character has perks that further increase survival odds and give a decent mob damage bonus if you aim well. Special weapons differed each level, they came in handy for bosses and larger numbers of elite enemies but otherwise run the risk of being out of ammo in a pinch without time to buy another.
In general, you should be aware that:

• You can use the ammo bag animation to get a quick look around while restocking from it.
  
• Decoy grenades are great for drawing enemies into a trap, but do not affect bosses.
  
• The amulet is perfect for catching a breath to wipe out enemy snipers as it affects all foes.
  
• Healing and armor breaking gems in your weapon of choice help a lot.
  
• Only the main game levels are relevant, not the DLCs.

The enemies that proved most dangerous were grenade throwers, bosses, fire breathing, and acid spitting ones, which is why they went on the target priority list in this order.

I hope this guide can help you enjoy Strange Brigade even more.
Please let me know if you find any errors or want to suggest some improvement.