I am running this on Normal with a Witch Doctor, the xp per quest is 10k. I tried it on Nightmare but the xp is only 5k so it's not really worth it to run on higher difficulties.
Most likely it's because of the kill radius and the fact that Kulle is supposed to die pretty fast. If Kulle starts to move around, then the bot won't move, with this you can't generate a path from point A to B because of the layout of the chamber (DB can't generate a proper path because the layout is not flat) and if you add more move points it will get stuck and do nothing, like in your case. On Normal difficulty Kulle dies in like 5 shots and there is no problem with it.
Most likely it's because of the kill radius and the fact that Kulle is supposed to die pretty fast. If Kulle starts to move around, then the bot won't move, with this you can't generate a path from point A to B because of the layout of the chamber (DB can't generate a proper path because the layout is not flat) and if you add more move points it will get stuck and do nothing, like in your case. On Normal difficulty Kulle dies in like 5 shots and there is no problem with it.