Black Guard pretty much nailed your guestion already but here are my two cents to add. Using bows against units with shields is usually not the best stategy. Atleast not whitout flaming arrows. Thunderstrike has huge fatigue cost. Even A4 mage gets 25 + spellcasting encumbarance fatigue thats 4 casts before sleepy time. Orblightining is only 3-4 + enc fatigue so that should give you more than 10 casts (in perfect world). Range for that spell is only 15 so you need to keep it in mind. Falling frost is other good option if you have constraction for the bracers.
And then there is always mammoths
