The great idea how to deal with the Zetars was to put the human they latched on to into a pressure chamber. I believe Mr. Spock said "if they follow the Captain's plan ..." I looked several times, but I could not find any mention of such a plan. They could negate gravity inside the chamber, and this had something to do with the fact that the Zetars were out in space for a long time. So what. Also, as a human, anyone might have problems if the pressure was increased rapidly. The usual scuba diving rules would still apply to a human. You can go down at a reasonable rate, but coming back up can cause a person to act like a soda can that was just opened. You must go up slowly and wait at several stages on the way up. Lt. Romaine was floating with no gravity in the air pressure chamber and the pressure was doubling rapidly. How did the Captain and Mr. Spock ever get that idea ? It seems that a scene was left out.
The other issue was the kind of brain wave pattern which was "identical" for the girl and the Zetars. If you looked at such a pattern for a human and recorded it again a minute later it would have a similar but not identical wiggly pattern. Even less likely if you compared it with an alien.
The other issue was the kind of brain wave pattern which was "identical" for the girl and the Zetars. If you looked at such a pattern for a human and recorded it again a minute later it would have a similar but not identical wiggly pattern. Even less likely if you compared it with an alien.