I'm not a guy who is big on PVP and i'm not on a PVP server so i never figured i would get my city defender achievement, though today i discovered a very easy way to get it if you have a ranged AOE.
The way to do it is to wait until IF is under attack then go to this spot pictured behind the kings chair before flagging for pvp.
then flag for PVP and AOE away wherever the horde are holding the King and get your achievement
Zee proof
Happy Killing.