Jörmungandr The world serpent.

[Vidmaster7]

So my party is max level almost max mythic rank. they have been doing stuff with Thor. I am about to have the face off with Thor and A snake big enough to wrap around a world. I want the party to have some part, whether that just be survival or what have you. I have some things that will happen in mind like Massive earth quakes and ridiculous poison. I'm trying to think of other things for them to do while the fight is going on and what they can contribute. I want the fight to be a event and I'm drawing a bit from the Death Wing World of Warcraft fight as well. anyone have any other suggestions about a fight with a creature that is so big one of it's scale would probably be a map?

[Bjørn Røyrvik]

Fight nasty symbiotes/parasites/offspring that infest the serpent?
Any number of big nasty critters that could fit.

Treat the serpent as terrain/weather rather than a creature?
Apart from making checks to stay on/near J, saves against poison, etc. they can make checks to hinder J. Maybe things like mythic Earthquake could injure/hinder it

[Azothath]

Jörmungandr at wikipedia as fantasy write-ups may obscure the legend the etymology is interesting and 3 stories are given. I think it helps with the symbology so you can set better goals, challenges, and plot. Essentially Jörmungandr & Thor must die to usher in a new age/world cue the wagner background music, Götterdämmerung.
Jörmungandr at world history

[Pizza Lord]

I haven't been able to find any reference in my searches, but I remember a Civilization-esque game where there was a Midgard setting.
Jörmungandr (his mouth) was a location where you could enter an underworld like system. It also spawned winged creatures like a monster lair (and they were inside him as well). These flocks of creatures were called
Jörmungandr spittle, basically like winged bat creatures that probably were also poisonous.

So you could also have him belching forth swarms of such creatures and individual such creatures of slightly larger size for the party to fight or to deal with so Thor can keep focused. Assuming the PCs can even hurt
Jörmungandr, maybe any slashing attacks also release such spawn.

[Azothath]

Widow's Kiss as injury/ingested with tiny snakes.
Purple Worm Poison
Dragon Bile
Swansong Poison homebrew

[Tim Emrick]

Take some inspiration from the kaiju subtype, particularly the massive quality. Jormungandr is on a scale beyond even that, but it could be a useful starting point for determining how the PCs interact with creature-as-terrain.

[Bjørn Røyrvik]

Please note that the name of the beastie is Jǫrmungand. The -r is merely the strong masculine nominative suffix, and not part of the actual name. I can only assume that the reason people insist on keeping the -r is because they look up the name in a dicitonary and don't consider grammar.