Question about Odin *Spoilers!*

Odin's power has always been something of a problem. He is so powerful its difficult to threaten him or Asgard. Writer get around this by having him unavailable like in the Odinsleep, water down his powers, or have a major threat.

In this story one thing emphasized was in his battle with Surtur Odin was never hit. He was able to block or avoid all of Surtur's attacks to wear him down. In this film he was caught off guard and hurt pretty bad in the first attack. That weakened him throughout. The commentary also confirms that the sword was doing a lot of the fighting and able to see through Odin's illusions. The sword was of sufficient power that it could kill most thing in one hit. One could argue it is a testament to Odin's power to last so long. Algrim was basically a weakling who was fighting a guy that could only be hurt by the magical equivalent of a mega nuke and Surtur's sword just happened to be one.

I think Odin was also watered down some for the story. Odin summoned a storm and some lighting but did not do anything with it.

I agree with the others but I would also add that I don't think Thor really wanted to hurt or kill Algrim. He had essentially taken him into his family for years and entrusted the care of his sons to him. Algrim gained a terrible power and kind of went mad, understandably so. Odin would prefer to reason with him in hopes that he would stock the fighting and they could go back to the way things were. Obviously, that didn't work out and getting hit by a weapon like Surtur's sword began to take it's toll on him.