Cru Z3R-14

The Cru Z3R-14 is a rifle with heavy damage and great accuracy, making it an ideal weapon for sniping. It's the hardest gun to get in the game because you have to defeat Reaper, considered the boss, or mini-boss, of the game because of it's high health, damage rate, and rarity.

How to obtain
You can find a strategy guide in the Reaper page of the wikia. Start from there. Then go into a safezone, preferably Lakeside Safehaven, and trade the Reaper Blades that the Reaper dropped for a Cru Z3R-14 (the first gun on the list).