To obtain this powerful pistol, reroll campaign until you find Hidden Grotto, a sewer dungeon where a female hunter coughing and sitting on the ground. Get her key and make your way to the end of dungeon, the pistol lies within.
The pistol can one-shot most minions aiming on their weak spots.
I finally found it after hours of rerolling.
![](https://s2.save4k.ru/pic/gSGvOMJQbPM/mqdefault.jpg)