Sword of the Necromancer

There are exactly 20 different types of monsters in this game, so for this achievement, you must defeat and revive every one of them. Like other achievements in Sword of the Necromancer, "To catch them is my real test" will only unlock once you reach a boss room or go back to the altar (either by dying or choosing to go back through the menu).
I recommend you kill and revive every new type of monster as you find them. Since this is a roguelite, you may have trouble finding a specific monster, but don't worry about that, you can keep replaying all dungeons floors after the story end. This achievement is not missable!
You can keep track of what monsters you have killed and resurrected by checking the Bestiary inside the Library. The Library is located on a bookshelf on the left side of the altar, below the stairs.

Here is the list of the monsters:
Slaaim
Fleye
Scholoak
Dollahan
Harvescalp
Champimbomb
Restorebbit
Skeletant
Heelion
Tai'ak
Trolks
Whispike
Mimigate
Sprinsioner
Kaogyps
Pobawa
Buffary
Impcher
Crawlumbra
Skullcrusher

After completing the game you can go through the right door and choose the "dungeon builder".

While building the dungeon you can choose what's inside the chests like weapon types/armor etc. and you can choose what kind of monsters spawn with the button.

Just look at the libary what weapons and monsters are missing in your collection and use/revive them.