[Finalgenesis]

I understand it as only 1 gem usable to raise spell level, the rest will each halve the fatigue used, total gems used up to caster's path level.

I think if you're above 50+(?) fatigue the AI will burn as many gems as it can casting spells. I always calculate gem use as using to his max path level in gems when casting spells at 50+ fatigue (or if a spell will put him over 100+) and give him gems accordingly, or basically give him the max possible gems he can spend.

It's hugely inefficient as we've all come to expect from battle casting AI, but at least you can work around it. In your case it would be easier, cast phoenix before sphere, you might pass out for a round or two before getting to fire storm if you only have 1S.