[Endoperez]

Lots of longdead would give your mages time to kill the Father of Winters with spells. Darkness makes him much less powerfull, Rigor Mortis makes him fatigued very quickly, and Drain Life can damage him.

Many Fire combat spells (mainly Evocations) are armor-piercing, and most Air and Astral spells (former in Evocation, latter in Thaumaturgy) are armor-negating. Both would work well against the Father of Winters. Your Cyclops could probably cast something like Thunder Strike and so deal him severe damage. Petrify (Alteration/Earth spell that either kills or paralyzes) would also work very well.