IIRC, you'll need to use a gun with a fast shooting rate, and you need to be in a specific spot to get all the shots in on time.
Took me a good 34902879238746958273590824312749587364587356 tries before I got it, and I think I borrowed Insom's Red Dragon Pistol for a short time.
Thanks for the quick response, I thought the pistol I was using would be fast enough, obviously not. I will try again later after some further exploring.