[fantasma]

look at the debug output viewing the battle. Run dominions in a terminal with -dd or so, more d's = more output. It says why the AI chose the spell (or not), well not exactly, but it gives hints.

I did this some time ago because it cast antimagic instead of master enslave, (antimagic was scripted by the next master to also effect the acquired units). The system works by assigning points to each spell, giving bonus for being scripted, number of targets affected, malus for excessive fatigue, etc. It simply ended with a lower number for master enslave than antimagic. My own army was too big compared to his, I guess.

You have to look what it says for you.