This is the Lessons and Bones quest. To find each piece, here’s a video guide. Each piece must be acquired at night:
Now, once you have the armor, you’ll need to generate a combo of 100 void stacks. Each magic attack can yield between one and three towards the combo, depending on the power of the attack. However, there’s a large drawback to the armor: Every attack drains health. After a dozen or so uses, you may find yourself stopping to heal, which can interrupt your combo.
Here’s how I did it: I set Mark of Flame as one magic ability. I made sure that the D-pad was set to quickly access health potions. I then fast-traveled to Vekreth’s Maw, where you spawn on top of a small platform. In front of the platform are an ice bear and two Barghests. Without moving, I started spamming the Mark repeatedly, but did not detonate it. It barely damages the enemies, so building to 100 was easy. I would tap my D-pad whenever I was low on health and use a potion. Once it got over 100, I detonated it. After the combo faded back to 0, the achievement unlocked.
The only drawback was it took a couple of tries because the bear would rush me and interrupt the combo. I reloaded and tried again, and it worked. I went back later after they respawned, and it worked again.
I suspect it could work on almost any reasonably strong enemy, as long as you can sustain the combo. Let me know if it does, along with if my solution works or not, and I’ll alter or update this as needed.