Agreed Shuddering Hollow is bugged af, but with that said this worked for us:
1. Contribute to Shuddering Hollow and get it to open.
2. Pull boss to the mouth of the cave, far enough to be beyond the rocks that close the entrance but not so far that he runs back in and resets.
3. DO NOT KILL HIM. Wait for the event to reset (rocks seal the entrance again).
4. Hold the boss there and contribute to the event again to make it reopen.
5. When the 2min30sec timer starts counting for the event to reopen, you now have the bonus objective again and the boss is outside the cave entrance.
6. NOW kill him, collect your gem.