To unlock the Pistol Whipped trophy in Anthem, you will need to complete three of the Heavy Pistol challenges which can be found by opening the Menu > Challenges > Weapons > Heavy Pistol
You will find four challenges available -- one for each of the game's three Heavy Pistols, and a final one for completing all three of the Heavy Pistol challenges.
The three challenges you will need to complete are 50 kills with the following three weapons:
Barrage
Resolution
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