I changed an enemy with FFTA AIO editor. It was a thief and i transformed it into a Paladin with defense. But the Paladin didn't cast defense a single time, even when it didn't have an enemy near. I wonder if i can tell the unit to cast defense everytime he can't hit in some way.
All units in FFTA use the same AI as far as we know. What makes a White Mage enemy act differently from a Fighter is what moves/abilities they have. Each A-Ability has a pointer for ability type and a value for the priority to give that move. "Life" for example, is a healing type move with high priority (70 or 100 I think) Which means if the AI can use life on a dead ally it will.
Some moves are set as player only, meaning the AI will not use these moves (all of the morph abilities fall into this category).
The default settings for the move "Defense" are: Use when at low HP, 10 Priority.
This means the AI will only use defense if at low HP and there is no other option (like attacking or using a potion)
Change these two settings till you find something you like.