You can find this Weapon early in the Game in the Level " Lost Tunnels". After you encounter your first Trapwire you encounter an Room full of Bandits clear the Room Pick up the Weapon and keep it.
Now the best Level to get both kills for this Weapon is the Level " Cursed" Khan will tell you that you have to Pick up a Bomb to Blow up the left Tunnel but first follow your Objective Marker and Blow up the Right Tunnel (Upstairs). Now follow the Objective Marker and Pick up the Bomb follow the Marker again to find the left Tunnel but don't blow it up yet. Stay right at the Beginning of the Tunnel and wait for an Infinite Pack of 3 Nosalises and keep Killing them and get all the Weapon Achievements you can get.
If you're going for Knife & Alternate Fire kills for the Volt Driver be Carefull! If you die you'll lose all your Progress towards the Achievement since the last Checkpoint!
Volt Driver kills don't count as "Pneumatic Kills"
Oh and btw...English is not my Native Language:-).
If your Achievements won't Unlock try this.
Quit to the Dashboard > go to My Xbox > System Settings > Memory > Select your HDD but PRESS Y instead of A >
Select CLEAR SYSTEM CACHE and Press A.
Now Delete your Metro Pack and Download it again. Start Up the Game and the Game Ask you if you would like to Update your Game , Choose Yes..