MAY BE GLITCHY...
There is a serious lack of information about this game in general (that should be a warning...) and this achievement in particular. I don't like the game nearly enough to provide a full solution but this achievement glitched on me so I'll post some basic info here that might help to avoid any problems.TLDR: Make sure you kill the dark enemy in Eastgate - don't let it just fall off the waterfall.
There are 8 "dark enemies' in the game. They are not bosses - rather you can find them waiting in the different regions. They can alternate between the different shields introduced through the game once attacked, so when you start they can seem virtually impossible to beat. They will be fairly obvious as they are glowing purple. If you're not sure and you come across an enemy that's kicking your butt, it's probably a dark enemy!
If you kill a dark enemy, you receive an essence. They appear in your Collection inventory as a spiky blue ball. They are also shown on the game map - you can see the regions that have a dark enemy and whether you have collected their essence. There are no special tricks - just kill them to receive the essence.
You have to progress through the story quite some way before you can get to them all. This guide won't mean much unless you have played through a decent amount of the game - I'm assuming you're coming here late-game for some help. If you're just starting out, don't worry about this too much, aside from the glitch warning below.
The dark enemies can be found:1. WESTCASTLE RUINS:
Fairly obviously in the centre of the junction area, waiting in the shallow pool. This is a Skeleton Mage. This will be the first one you come across and I believe you will see a pop-up about dark enemies at this time.2. TEMPLE FIELD:
Make your way down to the river and head to the east end. You'll need fire-run and spirit-form to get there. Just to the north of the archer you come across, there is a sloped block you can fire-run from. Run to the platform at the end, then jump up and use spirit-form to get through the gate. The special monster is past the 2 regular skeletons on a lower platform. This one should be fairly easy - it's a Skeleton Archer. There is a golden key here too.3. SOUTHERN BRIDGE:
Make your way down to the water. Head downstream and jump out on the left side. There is a Giant Skeleton just around the corner. Watch out for a couple of the small worms that hide during this fight.4. BALMY BEACH:
As you head towards the boat/chain down to the wintery area, this guy will be waiting in the shallow water between the land and the rocky outcrop Biff's boat was waiting at. It's a regular Skeleton that spams spinning attacks. For some reason, this guy was particularly difficult for me.5. BREAKWATER FALLS:
Just head all the way up-river. This Skeleton Mage is waiting on a small platform out on the water. 6. PEAKY PEAKS:
This is a Giant Skeleton waiting in a pool. If you fast-travel to the area, head into the main area and drop down to the left, as if you are heading towards Iceberg River. As you head up the hill, move to the right side to find it waiting in a pool over the chasm. There is a Slime in the water here too.7. EASTGATE
This one is well hidden and you will need to progress quite some way to find it. There is a lower section of Eastgate. You can access it from the top using spirit-form. Easier is from Temple Field - as you proceed towards the transition to Eastgate, there is a spirit-form gate (to your left after you go down a slope). It's basically opposite the Eastgate transition.
Either way, get onto the land on the side of the water and head up-river (you should be near the mini-tower). You can jump behind the waterfall into a small cave. The dark enemy is in here.
This is one of the regular Skeletons that spam spin attacks. The cave is quite small but just be patient and kite it around, using your special attacks to whittle it down. GLITCH WARNING - DON'T LET IT FALL INTO THE WATER OUTSIDE!8. REGALIA PORT CITY:
This is the most convoluted of the dark enemies as there is a side quest to complete before it appears. Once you have spirit-form, enter the graveyard which is on the path to Valy's house. In there, you can start a quest to help a guy who want to revive his friend.
This is a multi-phase quest. To start with, he wants bones. Initially 1, then another 2, then another 4 or 5. I went to Westcastle Ruins - the Giant Skeleton sleeping near the central pool (where you encountered dark enemy #1) almost always dropped a bone. If you burn the vines and transition to the north, the Skeletons in there would often drop another. Then head back and the Giant Skeleton would have respawned. Rinse and repeat.
With the bones done, he wants 4 monster goo. I travelled down to Peaky Peaks for this - the white slimes that split up are a good source for this.
Finally, he wants life - eg. 3 essences so you will have to kill at least 3 other dark enemies before you can complete this quest.
This should complete the quest and revive his friend. This is a large Skeleton that spams spin attacks, so it can be a challenge!
So, I did all of these. I had 8 Monster Essences in my inventory, but the achievement didn't unlock. I tried all the usual - delete/reinstall, etc... Of course, it may have been a random glitch but I have a suspicion that #7 was the cause.
When I first encountered this guy, I was low on health and healing options. I retreated outside and dropped into the water. The dark enemy followed me. The water is flowing hard here and we were both pushed downstream.
Eventually, the dark enemy fell off the waterfall at the other end, "apparently" awarding me the monster essence. But, I didn't actually kill it. I also defeated the Regalia Port City dark enemy last but I don't think that should be an issue.
Believe it or not, I replayed
the game up to point that I could get to all the dark enemies. I made sure to defeat the Eastgate dark enemy inside the cave. Just to be sure, I made sure the Regalia Port City enemy was not my last one. I also killed them all in one, single session.
To my immense relief, the achievement popped. (If, god-forbid, this happens to you and you are prepared to restart the game, I deleted all save data [local and cloud] and played right from the start in a new game rather than just choose a new save slot).
It stands to reason that Eastgate was the problem. I'm not sure if this warrants an achievement flag - please comment if you have any input. If it's glitchy, I'd like to mark it as such.
Oh, and if you're wondering, the essences are a completely useless collectible and can't be used for anything.
I've now completed the game but I really did not enjoy it. As I said above, there is not much information out there so hopefully this may help you. Good luck!