Gaining permanent control of a Chryssalid/Tentaculat
In order to gain permanent mind control/molecular control over a Chryssalid or Tentaculat, mind control/molecular control a zombie and shoot it.
The tentaculat/chrysallid that will appear that turn is hostile, but don't kill it. Next turn, and for the rest of the mission, it will be controlled by you without additional mind control actions.
If you lost a soldier/aquanaut to make the zombie he will be back after the mission if the controlled alien survives.