Originally Posted by
kombustion
I won't tell you how to do it, because I haven't tried it multiple times, but I'm sure there is a glitch as it's happened to me once. I ran the instance with a random dungeon finder group, and after the last boss, I alt-f4'd out of the game and went to bed. Upon re-logging in later that night, I found I was still in the instance and that the last boss (only the last boss) was alive and ready to be killed again. To make sure I pulled it and started working on, but didn't have the means to solo it so after I died I left and went about my business. My guess, is that after you finish the last boss, logging out while still in the instance possibly the group for X amount of time causes the last boss and only the last boss to respawn, but again, I didn't test it or try and replicate it as I didn't need loot from there and didn't really care.
This is in reference to Gate of the Setting Sun, I haven't experienced the last boss respawning with other dungeons, but again, It's not something that I've tried to replicate. It would need more testing to verify the amount of time needed to respawn the last boss, and what all instances it can be done in.