I was running around with 2 of my friends when I got this achievement. I used my Disintegration Ray on 2 Ghoul's and I received this achievement.

What is wrong? The achievement indicates that I have to kill 20 Shadow Vermins, Ghoul Wraith's or Shade Stalkers at once.

I believe when I got the achievement, I had only killed roughly 20 by that point. Either this is a bug, or there is a typo in the achievement.
Edited by Torazo#1738 on 5/17/2012 2:30 PM PDT