I Attack the Darkness

If you are looking to solo this, and you are a stubborn player like me who does not play a lock or dh and who refuses to ask for help from one, then you may be happy to hear that it is in fact possible to solo this with any class. Albeit, with a bit of extra prep and challenge.

It requires getting this item from Stratholme from the Nerubian boss in the Baron Rivendare side:

[item=13508]

This functions like Eye of Kilrogg and allows you to see invisible mobs, including the ones needed for this achievement. The only caveat is that it is unique and has only 3 charges. That means that after getting one, you have exactly one shot at completing this achievement before you have to go all the way back to Stratholme to farm another one. It is possible to farm multiple by letting the extras go to your mailbox via postmaster, however.

I'd recommend watching a video to find exactly where the adds spawn. Position yourself so that the add will move in front of you, tag the boss, turn your character to face where the add will be and then use the eye. Once you've used the eye you'll have to tag the mob with a ranged ability which will then break the channel for the eye. You won't be able to reposition once you've used the eye, so if the mob doesn't move in range and in front of your character then you will not have a chance to tag them while the eye is out. Tag and kill the add. Then just reposition yourself to a point where you'll be in range and facing the second add, wait for the 1 min cd on the item, get the boss to 65%, use they eye and tag and kill the second add. Then wait for the insanity bar to fill because it will teleport you. Move to the location for the third add, wait for the 1 min cd on the item, get the boss to 30%, use they eye and tag and kill the third add. Then quickly kill the boss before you get MC'd.

Did it myself as mage. It took three attempts.

Hey guys, maybe a bit late now, but my guild was doing this tonight and i discovered that my Warlock's Eye of Kilrogg could see the guys from unlimited range!

The Demon Hunter in my group could also see my Eye, so i i just called out when i could see it and a general area, and then he came over to grab it.

To get this achievement, as written must kill a creature in the dark for each of the three phases. First Phase is 100-65% Second Phase is 65%-30% and the last phase is below 30%. The creatures in the dark spawn in the darkness surrounding the bosses room. To find these we had a demon hunter use spectral sight to find the creature then hit them with a glaive toss to bring them into the boss room.