You can use non-retail weapons to get it. I used the hacker contact beam and have never bought the original contact beam.
Mine was having issues unlocking for a bit. I was using the seeker rifle when I managed to unlock it, which I know I had a kill with that one as I had obtained the Peek a boo achievement earlier, and went on and saved without dying. The only difference was, I had ALL my kills with the seeker rifle while using the alt fire's zoom. So that might contribute to it, not sure.
There definitely is something a little off going on with the achievement, that's for sure.