What is the best strategy to complete the "Hurly Burly" challenge?

The "Hurly Burly" challenge states:

Shoot bullymong-tossed projectiles out of midair.

I have successfully shot a few, but I find it very hard to hit them and most of the time I just get a rock to the face. Is there a weapon or strategy that is better suited to this challenge?

The easiest way to deal with this challenge is to stand in front of the Bullymong at around the range where the rocks will hit the ground, and keep shooting. The rocks are going to come at you anyway so it should just be standing around, pointing and clicking:

Here is a video guide on YouTube demonstrating this, in this video he uses a vehicle turret but pretty much any weapon you can aim with should work fine for this.

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.

A bit late, but I found a much more ammo-conserving way. Go to Three Horns, spawn a machine gun runner. Find a pack of bullymongs and kill them all except for a slinger. The slinger will retreat to a safe distance and start throwing rocks at you. Just aim at the rocks and keep shooting until you've shot 250 projectiles.

Occasionally, the slinger might move around, but their patterns are fairly predictable.

Try using a torgue shotgun, I found it a lot easier to complete during TVH mode, as most of the bullymongs are Highlander Type (A stronger version of bullmong that throw up to two rocks at a time, some throw Snow Boulders)

Strafe Left and Right to avoid the rock but aim diagonally with shotgun opposite of your strafe and you should be able to hit it.