You must complete three two star GRA challenges. The easiest (in my opinion) is to make a Cowboy perk guns based character and get the following challenges:
Talk About Owned: Kill Benny with his own gun. To do so, go through the main quest as usual. When you go to meet Benny, bring a 9mm pistol in good condition and some ammo and get him to meet you alone. Use the hostile dialogue choices and start a fight. Use VATS to shoot Maria, his unique pistol out of his hand, quickly pick it up, repair it with your own 9mm pistol, equip it, then VATS his head until head is gone.
White Line Nightmare: Kill 20 Fiends, Vipers, or Jackals using tire irons, baseball bats, lead pipes, machetes, .44 magnum revolvers, or sawed-shotguns. To do so, grab yourself a .44 magnum revolver (or the Mysterious Magnum), plenty of ammo, and visit Vault 3 with the .44 magnum revolver of your choice in hand. You should have plenty of fiends to shoot.
Dyin' Aint Much of a Living: Kill 5 members of NCR or Legion hit squads using weapons affected by the Cowboy perk. As you progress through the main quest, you should be getting hit squads from one or more faction. If not, kill members of the NCR and/or Legion to get their attention. If you are doing as previously instructed and playing a Cowboy perk based character, the challenge should be completed naturally as you defend yourself from the hit squads while doing other achievements.