This weapon is wielded by the 1st human enemy in the game. As you leave the ops center he's lying on the ground as you follow the path to the left. You need to cut his right arm off. If you miss it try on the next one most of them here use this weapon.
Once you have it play on until you get to the boss arena , it's after you follow the tracks around climb the stairs on the left and down the other side, at the back of the big open area is a door. In here is the dark zone to the left and down the stairs. Kill an enemy here with the ASTir SpecterBite and the achievement is yours.
There are other areas you can get this but this is the earliest.