The story started with a ninja trying to defeat a viking. We just kept finding ways to step it up, because neither of us wanted to be the loser in the series of battles. We'd make up silly reasons. Eventually the reasons get complacent, you need to find more grandiose ways of stepping them up and more creative ways of why you can't be defeated or why things happen. Ass-pulls like this happen all the time. Who cares as long as it's consistent, makes sense, and is fun.
This is the result of our story:
A powerful and unstoppable ninja seeks vengeance against the House of Odin. He takes the fight from Earth to the gates of Heaven where he infiltrates the powerful House where he destroys the entire bloodline one son at a time. Finally he faces Loki, and eventually Thor. After destroying both of them in the Colosseum, he takes on their father Odin himself. Odin proves a difficult foe. He becomes more powerful as he strips off his armour to reveal himself in the flesh. Odin kills the ninja, but this causes his true power to awaken. The ninja is now clad in dark, deathless armour. Odin is defeated. Their souls ascend to the final battle. In the higher dimensional realm, ninja strips off his armour to reveal that him and Odin are one and the same.
There is a plot twist. Odin and Ninja are brothers. Odin was adopted. Ninja should have been the true ruler of the Nine Realms. Odin reveals himself to be nothing more than the son of a long line of treacherous pirate lords. Heaven has been under the control of villains since the very birth of Ninja and these plans were set into motion long before he was born. Ninja wants to reclaim the throne, restore order to Heaven, and reclaim the dignity of his royal family bloodline.
The only way Ninja can kill Odin is with the Spear of Destiny. Their battle takes them back to Earth and before he can land the final blow, Ninja is vanquished. Odin goes back to Heaven to rule. Ninja wakes up to find that he is not dead.
He returns to Heaven to finish what he started. Only this time he must fight his own soul. After defeating his own soul, he is now a complete person with an equally powerful power boost. Of course he was never able to kill Odin the first time: Odin was in possession of his soul from the very start. This is also why he could not die. This is why he could not finish the job.
Ninja murders Loki and Thor, finishing the job he started. They are no match for his new powers, unlike the previous decisive battles.
Ninja faces Odin in the colosseum and easily defeats him. All of Olympus witnesses the defeat. Before Ninja can speak the truth, Odin's grandfather intervenes to fight Ninja. Odin's grandfather is a pirate as well. He vanquishes Ninja, forcing his awakened form. Ninja kills the grandfather but is shocked to find that he returns without a scratch. That is because he is possessed by an evil time travelling spirit that is the true mastermind behind the events in the story. Ninja kills the grandfather again but jumps into the time vortex before he can come back.
The final battle takes place in the time stream between the Evil Spirit and Ninja, who is now armed with the spear of destiny, his reunited soul, and the combined powers that the souls of his entire bloodline has lent him. The souls of his entire timeline of the house of Olympus - all of his forefathers and future descendants - converge into him to grant him the ultimate power of Olympus, making him into Odin the True God King. Unlike the previous Odin, whose armor was dark, this Odin's armor is white as heaven.
Whoever is defeated will be erased from not just every timeline that has ever existed, but also from every parallel reality branching from this convergence point.
The final decisive battle is swordplay and gun action with bullet time, because the evil spirit is able to slow down time. But True Odin is able to anticipate the actions and counter with his own manoeuvres. He defeats the evil spirit, retcons the time stream, and becomes a mythical creation myth to the people of Olympus and to the rest of the Nine Realms, effectively becoming the God that his people had worshipped since the creation of the world.