Getting the wraithwing past that point in the room is, I think, the most central point of the entire puzzle, and figuring out how to do that is the hardest part of figuring out how the room works. Once you've got that, solving the room is relatively straightforward compared to this step.
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Hint #1 (pretty clear)
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If the wraithwing is brained, it won't move southeast like you want, only southwards, since the brain path says Beethro is to the north. So to get it to go South it has to act unbrained for a turn.
Hint #2 (clear but maybe a bit less obvious)
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Looking at the mimic chamber and the three orbs the mimic controls, only one of the three doors near the entrance to the wraithwing path can be closed at a time.
Hint #3 (spoiler)
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If you keep the crumbly wall at (21, 5) intact and you close the westmost door, the wraithwing will have no brained path to Beethro and will act unbrained, and you can get it to move diagonally the way you want.
It is then possible to get it to try to move southwest, but slide south due to the north-pointing force arrow. At that point, it's safe to make it brained again.
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Let me know if you want a victory demo or a better explanation.
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Quick links to my stuff (in case you forgot where it was):
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[Last edited by Chaco at 06-05-2011 11:14 PM]