[Saber Cherry]

Yeah, your problem is that the spells (FFTS + FFA) do a finite amount of damage to each target, low enough that normal (10 hp) units sometimes survive. Jotuns will rarely be killed. However, maybe if you cast FFTS 2x in one turn, it would kill Jotuns. I think the damage accumulates. And as Verjigorm said, you ought to do it when they are not in their own dominion.

As for fire storm... I've never used it, but I used Foul Vapors once (as Ermor), and it was up for a similar 20+ turns or so against a force of roughly 400 living units... and I was very disappointed at how few of them died.