I usually don't post on these forums but I feel like the devs may actually read this.

This current "return to territory" or deaggro system is very easily abused, as mentioned above, but in case they want more proof of how absurd this system is:

Soloing Elder Mosshorn: http://www.youtube.com/watch?v=vQScwTR746o
Soloing Uraeus: http://www.youtube.com/watch?v=EP_YtWJtLtQ

Blog article on soloing with this technique: http://kanican.livejournal.com/53736.html

They already mentioned they plan to remove universal ranged attack from most mobs. If they remove that but don't fix this this will become even more unbalanced. I don't really have any ideas on how to best fix this problem, but until it is fixed in some fashion, every endgame event is going to be a joke for players that can properly abuse the system.