So I'm playing an SP game on the lovely Faerun Adventure map. Thirteen Mighty AIs versus little old Lanka.
Would you believe that I'd never played Lanka before? Man, I'm enjoying those evil little monkeys.
Anyway, my nearest AI opponent turns out to be Niefelheim. The AI even picked a N9B6 bless, which is pretty decent. (Not perfect, because Niefelheim certainly doesn't need more Strength, but pretty decent.) And I'm playing with the Better Independents mod, so the AI actually did a good job of recruiting Niefel Jarls & Giants.
The Niefel giants were scary at first, but my F9-blessed monkey demons took 'em down. And then I sieged his capital so he couldn't recruit any more of them. The castle-storming battle was pretty funny, because the silly AI put his Green Dragon pretender in the middle of his remaining Niefel giants. Green Dragons are
not cold-immune. Needless to say, the pretender cast one spell, shapechanged to dragon form, and then fell unconscious.
My point is:
Fire. Niefel giants are vulnerable to fire. Either swarm them with F9-blessed sacreds, or roast them with serious Fire evocations. Melt the big snowmen!