Redfall

So far I've only come across 2 Shrouds on Dusk Difficultly. It is worth noting whoever Stakes the Shroud gets the achievement.

Shrouds are identified by having a purple glow around them and long nails. You'll know when you're in combat with a shroud as you'll be in like a purple bubble area. Shrouds also have 3 blue shields around them with small gaps in between which can't be pierced. I've found using a Stake Weapon and timing it to hit the Shroud between the gap in the shields is the best way to do a lot of damage. I've found I can hit the Shroud once for a fair bit of damage before they move.

Shroud 1: Redfall Common - Located on upper balcony in the Overton Theatre.

Shroud 2: Buried Point - Side Mission - Eclipse.
Once you've reached White Pines Trailer park you have to kill "Aspect of the Eclipse", and she is a Shroud. This is how I unlocked the achievement

There maybe more opportunities to get a shroud Kill, however I'm 20 hours in and these are the only 2 I have come across.

EDIT

Shroud 3: Buried Point - Completing The Blackwood House puzzle - Katherine is a Shroud, once you kill her you can get the achievement and her room key.
She is located in Direcliff area in a cave near Bladewell Ranger Station.

Shroud 4: Buried Point - Lost In The Fog - Story mission as you go up the hill to destroy Effigies.

Upper balcony location in the theatre room of the Overton theatre. Climb the ladder on the right side of the seating area to find this enemy.

at sedgewick safehouse the white sheet has missions, stop the bloodbags from filling up the crystal, once you have done that vampires will spawn in sone will have blue shields kill the 1 that does not to release the shields and the achievement will unlock.