When you step on the mimic potion, you get to place the mimic anywhere you want. Move it to a good spot somewhere across the wall, and press 5 (or your key for waiting a turn). After that, the mimic will follow every move Beethro makes. It shouldn't be too hard to get it to all the roaches.
And I'm guessing the arrows you mentioned are in
Twice North, Once West. It is possible to go back through that room (and actually, you managed to go south past those arrows once before, when you beat that room).
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×It just takes a while longer than moving straight, that's all.