BB King

The whole achievement's text is wrong--you're supposed to skin them and bring back their pelts to Thrall. *nods*

Finished this quest this evening as a lvl 85 Prot Pally. First thing I did was to dump all my gear. I did Darn, Exo, Iron and Storm in that order. I used the following macro and completed each as follows...

#showtooltip
/cleartarget
/tar Muradin Bronzebeard
/tar King Varian Wrynn
/tar Tyrande Whisperwind
/tar Prophet Velen
/stopmacro [noexists]
/use Red Rider Air Rifle

1) Darnassus - Allowed myself to be killed under the platform that Tyr is standing on. Ran back and up the ramp and rez on the platform shoot Tyr and die.
2) Exodar - After exiting the ship go in the entrance for Exodar on your left, this will bring you very close to where Velen is. I was able to run right up to Velen before I was killed. Run back, rez, shoot him and die again.
*) Make you way to the eastern kingdoms...
3) Ironforge - Since the introduction of flight in the old world this is now a lot easier. I was able to fly into the center of Muradin's throne room before i was dismounted and actually got my shot off before being killed. Worse case scenario, fly in die, run back, rez, shoot, die. :)
4) Stormwind - Last and certainly not least was King Varian Wrynn. I strongly suggest flying into Stormwind from the northeast and entering the keep through an opened roofed "garden" that has direct access to the King's Throne Room. This one proved a little more "interesting" than the others since it is a very confined space. Eventually I had to rez, bubble, shoot, shoot, shoot several times before getting a hit.

A couple things to keep in the mind the range on the bb gun is 8-25 if the target is closer or further than those values it won't work. Another thing to keep in mind is that if you can accomplish this later in the evening in the middle of the week (Wednesday or Thursday) you will have significantly less people camping your corpse.

Either way, good luck, happy hunting and I hope this helps,
Malleosancti

You will receive a BB gun with 200 charges, once fired it will do 1 damage to the enemy meaning you WILL aggro the faction leader
Not usable while stealthed
Noy usable while mounted
Usable in druid forms
Must stand still while casting
Cast time seems to be 1 second or a little less
Range seems to be 10-25 yards
It can miss
You can use this gun any time of the year