Getting hooks on mobs

I'm wondering how I can get a livingentity object given a certain condition. My example is getting a hook on a mob when it enters an area so I can kill it with .sethealth. If there isn't a better way of doing it, how would I get an array of all mobs on the server so I can iterate over them to see if they are under the condition of being in that area?
--- merged: Feb 14, 2011 4:21 PM ---
Anyone? The only plugin I could find that does this sort of thing is WorldGuard, but as far as I know it doesn't have it's source posted.