Once a Winterveil achievement, you can now do this all year round. Buy the Red Rider Air Rifle from the Toy Shop in Dalaran and (if Alliance) 'pelt' the 4 Horde leaders with it. Horde players pelt the Alliance leaders. The gun comes with 200 charges and you can buy more if you want.
The gun misses a lot, has a cast time that is interrupted by damage, will always aggro the boss, and will occasionally backfire and hit you in the eye! :D
Needed help from Waylanderii (a guild friend) to get Garosh Hellscream as guards were even firing at me through the walls from outside because my aggro range is so high and the room is pretty small. I managed the other three bosses unaided. Big thanks to Waylanderii.
Very satisfying, silly, and did I say 'fun' achievement :) Fun FTW!
Listed on Joystiq.com as one of the 25 most evil achievements. lol
Music: 'Smoking Gun' by Robert Cray
Hope you enjoy :)
PS. If you do this on a high level char - you might want to take your gear off first - lol