I could be wrong here (I don't think I am though), but if you have a script that reacts to a particular elemental, and it happens
that the enemy is also weak vs that elemental, that script doesn't always execute. It's peculiar behavior and may need some
further examination.
Case and Point: Consider the Odin battle. Apparently casting lighting is supposed to cause immediate death to Odin, but it doesn't
quite work... and the success rate of triggering the script as far as I can tell at the moment is pitiful. The doll monsters summoned
by the Marions (FF2US name) react to all basic elemental spells (Fire, Ice, and Lightning). Apparently when their weakness is exposed
(which happens to be Fire), they seem to fail to counterattack with Fire.
It is interesting behavior that is worth pointing out.
Note: This isn't true if the battle was scripted.