I've tried many times through dec 2016 and Jan 2017 and the door is always closed. I'm a level 395 Hunter, I fly through avoiding all unnecessary enemies, no deaths on hard mode. Even on a perfect run the door is shut. This has to be bugged or there's some other prerequisite that hasn't been determined yet.
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chose the heroic modifier from the director.
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It was on heroic. We found it to be timed. I've since watched the door close. After many attempts I finally got it.