It's generally a good practice once in a while to list your battlemages on one side, list all the spells you have researched and they can cast on the other (note: if you gave them gems, the list is longer ! not only can they now cast spells one level higher, but all the spells that cost gems open up) and ponder "how are they going to cock this up ?".
What I do is figure the *worst* thing they could cast from that list, assume they will and try to minimize the damage from there, either through placement or gear or troop selection. Worst case scenario, I can always script 5 spells, then retreat.
They still royally bungle things once in a while, but that's because I'm bad at this
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