I think I've got a better way if you want to do this solo, but you'll need a specific legendary to do it.
I recently acquired a Nuclear The Dictator (element doesn't matter), and I noticed it came with a bipod. That helps because weapon accuracy is what kills you here with this chievo. Then, I shot this beast. 5 projectiles across a relatively wide spread, but in a perfect horizontal line!
You know what this means? It means hitting more than one target at a time is what it means! It also means that you effectively only have to be accurate vertically.
Keep at it! It'll still take a few tries, but it'll happen.
My previous best using many different styles and setups was 13... once. My first try with this puppy was 14.