×There appear to be exactly enough roaches in the upper right to fill up the "trap" to the left of that serpent surrounding the mimic potion, minus 1 roach to kill the serpent. So which orb opens their door? I assume it's the one next to that serpent...so I think you might need to stand on the trapdoor, strike that orb, and then wait for the roaches to get as close to you as possible before stepping off the trapdoor, killing the brain, and then luring them into the trap and then the last one kills the serpent. Then, hopefully, you can kill the other serpent and the roaches in the trap while stuck in that small room with the mimic potion...

×You can go much faster through the area before the trapdoor at 1. The roaches don't need to go past 5 yet.