[Baalz]

The thing is, fire is AP (or regular damage) while lightning is AN. With an earth bless in a cold dominion the giants have over 20 protection so even with their fire weakness you've got quite the uphill battle...plus AOE is less effective because theres only one per square. Lightning, OTOH, benefits by there being only one per tile (damage is concentrated) as well as the air mage prescison bonus. Throwing down ~20 orb ligtnings per turn and you won't have much trouble lighting up each giant several times in short order. Besides, you're generally gonna have to hit each one several times to make a soul slay stick as well...