You receive the Dreamstinger bow at the end of the story of the dlc. You will also get a few cards: two of which are a Dreamstinger bow skin and a fully upgraded Dreamstinger bow. This achievement requires you to confuse 10 enemies with the bow. The easiest way to do this is to go to Expeditions from the Main Menu --> Replay Chapter Elite --> The Tower, and play that mission. You can get The Wrath of Siberia, Persuasive Argument, and Servant of the Witch achievements by doing this also. By confusing enemies, the achievement is asking you to
*** Spoiler - click to reveal ***confuse enemies using the hallucinogenic pollen of Baba Yaga. This effect is actually the secondary poison arrow type (pressing 
to shoot the arrow), but instead of killing enemies, it frenzies them, making them attack anyone, even each other.
Repeating this mission once or twice will nab you this achievement easily, especially if you've already beaten the main game.