You should pick this up as you progress in normal play. However, the Brute Shot is not available until mission D-1. At the start of that mission, just wait a few moments for a Brute to show up and kill him. Pick up his Brute Shot and wait for the next one. Kill him, if you've used the other weapons this achievement will pop now - you don't need to finis the level.
Make sure you have killed enemies with the Needler. You get this as a secondary weapon on mission C-4 so head back there if you need to.
I don't recall buying the Sniper Rifle and I've seen on the Win 8 page for this that others haven't needed the Sniper Rifle either.