Sea Salt

There are 19 cards depicting the different minions/units you can use.
Most of them are not really missable as you'll have to defeat certain enemies to progress the story, thus earning their cards. But due to the fact that you cant replay missions in this game, you'll have to start a few new runs because of the path splitting up and the decision with the Witch.
6 cards are pretty well hidden in specific rooms.
Just pause the game and you can see the name of the room on the bottom left of your screen.

I will try to add some videos in spoiler tags to the guide, or find out the room names more specific.
Videos are added for all the hidden cards, if someone wants to count the rooms number (e.g. its the 8th room) feel free to tell me i'll happily add it :)

-----Ruins

The Swarm
You'll start out with it
The Worm
Unlocked during the tutorial
The Cultist
Defeat Admiral Pierce

-----Karlshaven

The Madman
When you take the EAST route in Karlshaven --- West construction block --- at the bottom right of the room you'll find two enemies standing on a dock with barricades instead of a railing, destroy the barricades. Follow the hidden route to find a drunk man and kill him to get the card.

The Spitters
When you take the NORTH route in Karlshaven --- Back Alley --- behind the sacrifice pit you saw earlier in a cut scene, walk to the left through/behind some buildings to the card.

The Crabs
Both routes --- defeat your first Vanderberg hunter (yellow coat)
The Drogher
If you took the EAST route --- defeat Warfmaster Glenn
The Fly
If you took the NORTH route --- defeat Deacon Raphael

-----Rotwood

The Creature
If you took the NORTH route in Karlshaven --- Old Mining Road --- at the start of the room theres a big cave at the top with some hidden barricades in it, destroy them. Go up and keep pushing right, you'll end up on a small patch of land with a card on it.

The Shaman
If you took the EAST route in Karlshaven --- Training camp --- at the top left there are two barricades between trees, destroy them. Go up and left to find the card.

The Lich
Both routes --- defeat the Lone Warden

-----Hogstead

The Black Cat
When you take the EAST route in Hogstead --- spare the Witch
The Drowned
When you take the EAST route in Hogstead --- defeat the Witch
The Flesh
When you take the NORTH route in Hogstead --- defeat the Thistle Guards
The Fishman
Both routes --- defeat your first Blavatsky hunter (yellow coat)

-----Nova Ugarit

The Toad
Both routes --- defeat Count Volgrave
The Locust
When you take the SOUTH route in Nova Ugarit --- Cathedral District --- At the bottom of the room is a sacifice altar with 5 dogs to each side. Destroy the hidden barricades in the water and move to the bottom right and east, you'll end up on an island with the card on it. (I had trouble with a big horde, after a few tries i eventually got into the entrance and could move east.)

The Maggots
When you take the NORTH route in Nova Ugarit --- West entrance --- just walk a bit down and to the left, the card is clearly visible.

The Hermit
Both routes --- defeat the Archbishop