By far the best solution I've found is to do it right near the start of the game, when you're in Liars berg and Hammerlock asks you to kill the bully mongs in the cemetary. The bullymongs aren't programmed to leave the cemetary so if you sit a short distance away they'll just sit there and toss projectiles at you endlessly, never attempting to close the distance. From there it basically turns in to the world's most predictable shooting gallery with you shooting each rock they throw and maybe runnning to the ammo machine in Liar's berg when you run out of pistol ammo. I've found it takes around 15-20 minutes to do all 5 ranks of the challenge. You can even use it to farm badass ranks if you make a new character every time.
Of course all this assumes you're a good enough shot to hit each rock, but it's so predictable that is really is extremely easy.