I had the plasure to see the battle by using a scout. Very nice as I just declared war on you
It seems to me that mages do not follow the script when the scripted spell has no valid/useful target. Here the problem was probably petrify which is a short range spell.
It happens quite a lot, sometimes the enemies are not there or are all protected - and anything can happen. An archdevil of my enemy summoned one huge elemental which put his fatigue way above 100 and effectively killed himself. All my troops were fire immune and he had some fire attack spell scripted.
I guess there is very little one can do. However, most very nasty spells need gems, so keeping the gems at only what is really needed might help.
Of course, then there is the problem with breath of winter, which seems to be the favorite of all water mages...
Turms