I just did this room a different way than described above, and due to my placement of the decoy, I didn't need to be fast. I also used the mimic to kill the snakes.
I put the decoy
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×at the top of the room to block the tar from becoming uncuttable from the top.
I put the mimic
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×in that little slot on the left to kill the roach queen.
Then my mimic
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×walked through the eyes and the bombs and the roach queens (killing them) and the tar (killing the mother) to finally kill the snakes.
To let the goblins out,
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×I had the guard strike the orbs in his chamber. I didn't need the mimic for this.
Hope this helps!