An ingenious attack method built right into GSIV. Cold and fire spells against stone creatures cause extra damage. Casting Dark Catalyst against stone trolls or giants causes their skin to crack to horrific effect. The temperature differential adds another round of damage a moment or two later.
This is different than the standard Fire Spirit against trolls. That merely slows or stops regeneration for a time. The temperature differential is extra damage. This is a good thing because that stone skin is as tough as metal.