What the achievement asks
Acquire 100 stacks of Unseeming Shift in the Maddening Deep. Five points.
The fast way
The Maddening Deep is in the south of Azj-Kahet. Fly to roughly 67 83, stand in the pool, and wait. The stacks build on their own and a hundred arrives in a couple of minutes.
That is genuinely the whole achievement. There is nothing to kill, nothing to click and nothing that can go wrong.
Two alternatives, and one to avoid
Black Blood Extractors are scattered around 62 82. Each one you click gives five stacks at once, so working through them is faster than standing still if you would rather be doing something.
Standing in the Black Blood at the Ruptured Lake also gives about five stacks a second, which sounds ideal. It also does heavy damage very quickly. You will not survive to a hundred there, and the Maddening Deep pool costs you nothing.
What the stacks are actually for
Unseeming Shift is not only an achievement counter. While you carry stacks you can see and interact with things that are otherwise invisible in this part of the zone: discordant creatures you can kill and loot, and at least one treasure.
The Memory Cache of Treasures of Azj-Kahet, at 62.73 87.90, is invisible without stacks and is the treasure people most often report as missing. Build your hundred, then go straight there while you still have them.
Where it fits
One of the pieces of You Xal Not Pass, which rewards the Shadow of Doubt mount and is part of Worldsoul-Searching, the War Within meta.