For this achievement you need to kill 50 enemies while wearing the Wraithskin outfit (witch you get as a gift for buying "Baga Yaga" DLC).
For some reason, farming enemies on "Score Attack" didn't work for me (And that worked just fine on Xbox One)
Fortunately, you can do this easily on Story mode, or post game: keep fast travelling between Siberian Wilderness and the Soviet Installation Camps and kill those random enemies that respawns.