Westerado: Double Barreled

You get a hat shot by shooting just above an opponent's head. You'll know it's successful because their hat will fall to the ground (which can then be picked up for extra lives) and typically the enemy will surrender and run away.

Both the rifle and the single pistol are probably the most reliable weapons to use for hatshots. The dual pistols are harder to aim, and can quite often result in a hatshot followed immediately by a headshot which will stop your progress for this achievement.

If you're having trouble finding enough enemies, or hitting enough hatshots while under fire, this can be done on civilians in either Clintsville or Santa Anna.

Edit: If you don't unlock this through normal play /u/WhyattThrash has an even better solution: "An extremely simple way to boost this is to get a second controller and bring in a co op partner. They have three hats so just pew pew pew, bleep bloop"