The simplest way to pull this off is to have the following build, as I had in the OP.
Human Wizard (Transmuter, banning Evocation and Necromancy) 5/Red Wizard (Ban whatever you want) 5/Incantatrix (Banning Illusion) 5/Cleric 1/Dweomerkeeper 4
Feats:
1st: Tattoo Focus
Human: Widen Spell
Flaw: Chain Spell
Flaw: Spell Mastery (Retrain to include Fimbulwinter later)
B: Scribe Scroll
3rd: Fell Drain
5th: Energy Substution (Cold)
6th: Flash Frost Spell
9th: Lord of the Uttercold
10th: Transdimensional Spell
11th: Enlarge Spell
12th: Uncanny Forethought
15th: Metamagic School Focus (Transmutation)
15th: Piercing Cold
16th: Extend Spell (Bonus).
18th: Arcane Thesis (Fimbulwinter)
Domains: Magic and Planning
Buy Iron Will at the Oytugh Hole before level 11.
Capabilities:
Casts Fimbulwinter as a standard action and a supernatural effect.
Any metamagic feat applied to a spell from the Transmutation school has its spell level adjustment reduced by 1. Metamagic can be applied for free. All metamagic applied to Fimbulwinter are at -2 spell level.
So, here are the stats of Fimbulwinter, with all metamagic possible, when cast at 20th level.
Supernatural Effect.
Duration of 100-144 weeks
Area of Effect: an 88-mile-radius circle
Effect on everyone inside: 8-10 feet of snow on the ground. Winds of 31+ mph. All creatures within take 8 cold damage, 8 negative energy damage, and one negative level per round. Cold damage goes through cold resistance, and is only reduced by half against cold immunity.
Hits Ethereal and Incorporeal creatures as well.
Spell level: 8 (or 20, for the lols)
Repeated 20 times per casting, with the center of each effect up to 88 miles away from the last one.
And you can make a scroll of this puppy.
Oh, and Altpersona? Fimbulwinter is already a cold subtype spell.