I can do it with all the roaches still alive, and without any roaches behind me when I get to the wall.
You can do it this way:
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× Enter the room on the square furthest north of the gap.
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× Now walk northwest, and then north to stand behind the wall west of you.
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× Make sure you trap all the roaches you possibly can behind the wall west of the potion, preferably into as tight a space as possible.
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× Pick up the potion and then step carefully to stop the roaches smelling you and head to the western part of the room.
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× Your last obstacle should be one lone Roach trapped behind a wall. Go far enough to the north to avoid it following you, and then the rest is easy.
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Resident Medic/Mycologist