[Verjigorm]

Jotun typically live in cold provinces. I believe this reduces the effectiveness of Flames from the Sky. FFTS, as I understand it, divides a given force in half and attacks only one half of them. Fires from Afar affects 10+ targets, so your mages with more than 3 fire books can get extra targets, but according to the description, it says that the chance of one of your 10+ fireballs hitting an enemy is dependant upon the size of the army (e.g. small armies are harder to hit) while Fires from the Sky hits all of the units in a given camp (e.g. 50%).

Try casting FFTS and FFA against someone other than the jotes--someone with small, low hp, massed units like T'ien Ch'i.