I made a video showing all world bosses you need for the hunting achievements and their possible spawn locations:
http://www.youtube.com/watch?v=vngB-fSe5YE&hd=1
The respawn timers (which are random, btw) are written in the description. The intro will also let you click on a thumbnail of a certain boss to jump directly to that boss section, if you're missing only a few and don't know where they can spawn. Or if you're missing most of them, you could watch the whole thing and enjoy the music 
Read the video description for more info if needed.
