So True polymorph can be used on inanimate objects to turn them into cr 9 or less creatures. After maintaining the spell for the full duration they become creatures permanently. Now for those instances when you know where the big bad is going to show up, and you know it isn't immune to petrification, medusas are incredibly handy. THis is by no means a single character trick, you will need some face/sociable party members to help keep the medusas on your side for the big bad fight. Wall of Force is invisible and provides exellent cover for the Medusa to care bear stare from.