For this achievement, you need to kill every single enemy type in a single run. You need to kill at least one of each of the following enemies, in increasing order of rarity:

• Zombie
  
• Chaos Agent
  
• Rat Pack
  
• Mutant Thing
  
• Slime
  
• Meat Machine
  
• Darkness

To be precise, you need to actually KILL these enemies - this means that skipping them by throwing a net, hiding, using a secret passage or any other method doesn't work. Anything that kills the enemy does work, including using the implosion vest on Darkness.

All of these kills need to happen in the same run; they are reset if you load a game. So you can't, for example, kill a Meat Machine, save, and then keep reloading to get other rare enemies.

Probably the hardest part of this achievement is generating a character with the hard difficulty that is actually capable of killing all the enemies and finding Darkness in time with a higher chance than 0.1%. I recommend the following:

• Endurance: not important, although anything lower than 10 increases the risk of dying from radiation.
  
• Wounds: not important, if your other stats are good. I did it with only 8.
  
• HtH: very important, having a high HtH will make it significantly easier to progress through the decks. Aim for 12 without Chainsaw or StunNux, 10 with StunNux or 9 with Chainsaw.
  
• Marksman: not important in the slightest.
  
• Reflexes: fairly important to prevent traps from screwing you over; aim for 10 or higher.
  
• Skill: Strength and Agility are decent, Climbing and Hiding are excellent. Climbing to prevent chasm cards from ruining your run and letting you skip cards late game, hiding for reducing the odds of strong enemies killing you.
  
• Equipment: very important, don't bother trying with anything less than 5.

Once you have rolled a decent character, you also need decent equipment. I recommend:

• 2 HtH items: hope for Chainsaw or StunNux, or some 1-hit kill item for Slime.
  
• 2 Clothing items: you absolutely NEED the implosion vest for Darkness. Don't bother trying without it. Since you will use the vest, you can use your Laser Lance to kill Slimes.
  
• I put the remaining points into devices - there are a lot of fairly useful ones like climbing rope, flare gun, metal / heat detector, wirecutters, etc. You could also pick a gun for blocked doors and strong enemies.

First, keep rerolling characters until you finally get one with the right stats and items. Then, BEFORE OPENING ANY CARDS, make a save. This is the save you will keep reloading if you fail.

Play the game as normally, until you encounter either Slime or Meat Machine. When you get one of these, make a second save on a different slot before killing it. This is another save you can keep reloading, with the advantage of having a guaranteed rare enemy.

Then just keep playing, doing anything you can to stay alive. Hide (but only if you have already killed the enemy), use guns, etc. Don't skip any cards using secret passage, flare gun or climbing rope until you have killed at least one of each rare enemy - that is, Mutant Thing, Slime, and Meat Machine. After you kill all 3, get to Darkness as fast as possible, skip as many cards as you can, etc. If you die at any point, reload either of your two saves and try again. Note that after reloading, you current card will be the same as before the load, but the rest of the decks get randomized, meaning you can reload even if you get a chasm or something.

When you find Darkness, immediately use your implosion vest and you will get the achievement.