I got that one on my first playtrough, without going after it or anything. I think you do it by just doing the "Peeper" quest and the Mordus cave quest (I don't recall the quest's names, but you should know which 2 I'm referring to).
It could be bugged anyway and I was just lucky, still, maybe it helps to know that someone got it. I am, however, having problems with some other achievements, so I'm pretty certain they're not all functional atm.
