This trick is quite easy to perform, even if it takes a while to gather all the necessary guards to escort you (estimated time is about 20 minutes).
It is all doable thanks to the mechanic they added a while back in Cataclysm, that makes some NPCs unable to have their HP reduced below a certain threshold when hit by other NPCs. This allows for example, the Ancient Protector or the invading horde NPCs during Tiragarde assault to be kited around and used to gather an army of hundreds of guards (those spread around Proudmoore Keep).
This can definitely be used for horde too but with different NPCs at different locations! Not many other locations are as practical as this one however.
The way the guard aggro seem to work is that if you are kiting them with an NPC that doesn't hit them or cause self healing, the horde you attack first will be their focus target as long as they got some kind of heal proc when you hit them, such as Impassive Visage.
