You get this achievement for beating the boss at the end of
*** Spoiler - click to reveal ***The Devil's Throat. I found this one to be rather annoying. Basically, the Chromatic Essence is just like any other Essence enemies; the big difference here is that once you kill it, 3 more will spawn.
For example, it starts with the Chromatic Essence, then turns into 3 Essence of Fire. Killing each one of those will spawn 3 Essence of Lighting, and defeating each of those will spawn 3 Essence of Rock, which finally split into the 3 smaller ones.
What I did here was kill the Chromatic Essence, which spawned the 3 Essence of Fire. Then, I tried to focus on one of those, turning it into 3 Essence of Lightning. After that, I worked on one Essence of Lighting to get the 3 Essence of Rock, and killed those.
I'll try to break it down a little better:
- kill the Chromatic Essence, which turns into 3 fire.
- kill 1 of those Fire, and have 2 Fire and 3 Lightning.
- Kill 1 Lighting, and end up with 2 Fire, 2 Lightning, and 3 Rock.
- Killed the 3 Rock (and their splits), then went for another Lightning.
- There would now be 2 Fire, 1 Lighting, and 3 Rock.
- Again, killed the 3 Rock (and splits), then tried for the other Lightning.
- After that Lightning was killed, then the Rocks (and splits), tried for another Fire.
Repeating these until all enemies are finally wiped out. This is easier said then done, especially since attacks can hit multiple enemies at the same time, and there really is not much room to run around. I just know the last thing you want to do is face 27 Essence of Rock at the same time.
Edit - Replaying this area with a stronger weapon made this fight way more easy. I used one of my lightning guns, and was able to kill them all with no issues.