I have been going here every now and then. I have been unable to complete it twice due to mobs glitching outside of map. The one that recently happened was when Nightmare and one other mob were left. Not sure if you can do anything about this since it might just be a glitch in the server files related to other mobs being created (white wiz/golden/etc.) Here is a screen i took:
Were any of the mentioned mini bosses announced on global during that time? Nightmare does have a predefined range of default coordinates and while it will spawn to a destination that is reachable in most situations I guess on rare scenarios it may glitch and spawn outside of the map. Have you waited the entire remaining time and he didn't teleported anywhere else on the map until time expired?
I don't remember anything being announced on global for bosses/mini-bosses...i usually try to time it so nothing is spawning (even white wizard). I didn't wait the entire time since it would waste the durability on the ring and by the time i would wait... i could almost have another run for nightmare going
