So... only the people who were in the dungeon right at the start will ever be able to fight those three bosses or get the achievement? And it was over in a couple of hours and is never resetting? (PC-EU)
The west and east zones are deserted. No monsters spawning or respawning.
The north zone... A few monsters respawn every so often with ballista parts. You can use them to repair those ballistas... but there's nothing to fire the ballistas at. There's a huge health bar at the top of the screen, stuck at about 40%, but no monster for it to belong to.
Went away, came back from an hour. Same was still there.
And there are no instructions from anybody, or misleading instructions telling you to do something that's apparently already been done.
If you're going to instance a dungeon like this, it needs to FULLY CLOSE (and throw out every person who is in it) when the big boss falls, and then RESTART FROM TOTAL SCRATCH. Or all the monsters in it, everywhere, need to respawn, and all the things-to-do (e.g. destroying crystals with ballistas apparently) need to reset REGULARLY. Like, literally, as often as in a regular public dungeon.