You confuse enemies buy shooting poison arrows with the Dreamstinger DLC weapon. The weapon and outfit unlock when you complete the DLC in story mode, but you get the cards right away in Expeditions Mode. You do not need to play a DLC area to get this achievement.
I replayed "The Tower" main story mission on score attack on any difficulty. (The Acropolis level has more enemies and is better over all but I completed it on the Tower). I used the Dreamstinger V, Wraithskin: Master, Toxicity, and Tomb Raider cards. Almost every area of this level has a few good spots to get 3 people with one poison arrow. I got it in the first area when enemies were dropping down. Just equip the poison arrows and cloud up the whole level. If things are too crazy for you, the last room has 3 enemies talking with their backs to you. You can easily get all 3 in one shot.
Here is a screen cap of the score attack setup: (larger version linked under the image) http://images-eds.xboxlive.com/image?url=imbJM3vwvcwNUnV76aR...
Here is a video clip of the end of the mission where you can get 3 in one shot. You will most likely get the achievement well before this part though.
With the Wraithskin outfit you should get these achievements in 1 run.
The 50 kills with the outfit might need a couple more enemies but you will be very close if you do not get it.