There's a couple of ways to do this one, with the intended solution using the room with two doors. I did it without. Don't worry about the mimics to begin with.
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×Open and close the three doors to manipulate the roaches so that you get them all together. I gathered them in the space between the middle and right doors. Then open the left door, and when all three roaches are in the room with two doors, hit the fourth orb. Now it becomes obvious where the mimics need to go. I didn't bother with the room, but opened the middle door and placed a mimic there to meet the roaches.