Fenrir... HELP!!

[OGDojo]

okay so i'm a fairly new DM and i decided to start a campaign based on stopping Ragnarok (well more like stall it but you get the idea) well the first boss was going to be Fenrir as it is he that eats the sun and kills Odin in the mythos. so my main question is... how the hell do i build him?!

im using Pathfinder rules to determine XP and CR.

Giant wolf (preferably colossal or bigger if there is such a thing)
special attacks
immunities
BAB
HP
Challenge rating i would like to keep under 20
i just dont have the experience of building creatures or finding their challenge rating. i mean if i built it myself i could easilly make it too overpowered or too underpowered, i want it to feel like a challenge without it being stupid.

Please note that i do plan on copying the whole campaign into a google doc and linking it so others can play it so you wont just be helping me here you will be helping the Pathfinder community. also there is no real reference to Fenrir as a creature that you can fight the best i found was a 5e monster on D&D wiki in the homebrew section.

so this is kinda a synopsis of what i would like

ideas on how to make a wolf colossal sized
ideas for attacks and stats
ideas for any templates (Instrument of the Gods seems to be a pretty legit way of making him unkillable)
ideas for any feats that you think it would have (pounce would be the one i would want most)
ideas for any immunities (poison, sleep, fire, acid?)
and a way for me to calculate CR so that i can judge when to throw the son of Loki at the party

if you can help, thanks
if your just here to put down my idea, *points to trash bin* you can file your complaints there.

Fenrir CR19
Colossal Outsider
Initiative +3
AC 33 (touch 6, Flat Footed 30)
HP 408 DR 20/epic Energy resistance 30
Spell Resistance 35
Str: 51/20
dex: 17/3
Con: 31/10
wis: 14/2
int: 12/1
cha: 16/3
Fort: 25
Ref: 16
Will: 11

Attacks: Bite +37 (4d6+20+4d6 cold+1d4 bleed) 20ft reach
Claw x3 +37 (3d6+20+4d6 cold+1d4 bleed)
Templates
Gigantean CR+7: turns the medium sized worg base, into a colossal sized creature
Advanced CR+1: gives a +4 to all stats
Chaosborne CR+1: gives a load of abilities and some stat changes
Cold Creature CR+2 gives some immunites and some cool abilities as well as a bunch of other stuff
Eldritch CR+4: a good way to give your creature anything you could really want cr is based on the abilities it gets
Savage CR+1: gives Dr10/epic and energy resistance 15
Invincible CR+1: gives Dr10/epic and energy resistance 15
Abilities from templates
Chaosborne
See Chaos
Aura of Anarchy
laws Failing
energy immunity: Acid
spell vulnerability: Dispel Chaos
Protection from law
Entropic Blow
Chaosborne Curse
gaze of discord
cold creature
energy immunity cold
cold aura
freezing blows 4d6
freezing embrace 20d6
freezing touch
cold body 1d6
cold walk
Feats (gains 13 feats from Hit die)
Still need suggestions on this part

[Buufreak]

well the first boss was going to be Fenrir as it is he that eats the sun and kills Odin in the mythos.

Actually, you can thank Skoll and Hati for eating the sun and the moon when the end begins. But hey, I'm not here to argue. Just watch out trying to discuss anything with real world inspiration. Someone is bound to get pissy and then a certain mod or 3 show up and lock **** down.


so my main question is... how the hell do i build him?!



Easy. Take wolf from MM. Apply titanic template. It is now gargantuan, and the template does pretty much all the math for you as far as stats changing. Then probably apply some intelligence, as Fenrir is known to actually speak (see the story of how Tyr lost his hand.) That should just about do it, considering this is only a "first boss."

Although knowing a bit about the party would help, too.

[OGDojo]

that is a great start! thank you!