Frustratingly Stupid Fire Mages

[Calahan]

Even though it sounds odd, you could try having less men on your side of the battlefield. As the AI is not casting flaming arrows as at the start of the battle, it is judging you have enough troops to win without needing to expend any gems. And is then only using gems later, after you have lost some troops, as all of a sudden the AI decides you do need to use gems to win, and so burns them on Fire Elementals.

So you can try to convince the AI that your army does need to use gems to win, say by maybe leaving out 10-20% of your troops, as then the AI should be more likely to use gems on round 1 of the battle, and hence will then follow your scripts instead of ignoring them.