A severe problem

[Novi]

Erm... So if I understood you correctly a mass of longdeads should do the job? (CR? Chaff army?)

[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.

[vfb]

CR 100% is the innate 100% cold resist of undead.

They are 'chaff' because they aren't there to hurt the Father of Winter, their purpose is to stop him from getting to your mages, who will do the damage with their spells.

Does he cast 5 spells then attack? Or just charge up and start hacking away? What kind of support does he have? And did you Spirit Curse him already?