(31,24):
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×This is the easiest. It should be obvious how to get a roach on the first pressure plate. To open the door to the second, you need to get three roaches caught behind the force arrow at (9,16) by going through the tunnels. This isn't very complicated. You need three because if you only have two, they'll go back to the southeast.
(8,21):
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×To get a roach trapped in the area, you just need to lead it to the southern wall with your sword NE.
To actually open the way to the plate, first you need to get a roach on (31,24). Then just let a roach step on the plate north of the door that opens.
(31,17):
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×This is the tricky one. You can get a roach caught on the plate while going through the tunnels, but when you go south it dies on the hot tile. Naturally, there's a trick to it.
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×Note that the pressure plate above the green door opens the door at (32,4) as well as the one at (33,5).
Of course, if you stepped on that plate, you wouldn't be able to leave.
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×So don't step on it yourself.