To have a female that cannot turn into a crystal or treasure box, you must have the Chantage perfume (stolen from Meliadoul in Chapter 4) and a character skilled as a ninja, or have a Vanish Mantle as an accessory.

Put the Chantage perfume on any female character, and make sure the Ninja has the reaction skill of Sunken State (or equip him/her with the Vanish Mantle)

Go into battle, and hit the Ninja until he uses Sunken State (unless he has a Vanish Mantle) then make your Ninja run away after it is transparent (not acting!)

You will notice that your female character, after dying will be revived as soon as her AT reaches 100. This will happen every time she dies! It is better to have someone like Agrias use this technique!