My dad mentioned a scene potentially from a cartoon. He thinks it's Heavy Metal, but I don't agree.

The scene is described as two societies at war (my dad says two separate planets). Rather than going at each other full-scale, each selects a single champion. The following scenes focus entirely on one hero as he trains in multiple forms of combat and is sculpted into a peak physical specimen. On the day the two are set to meet, the hero is boasting of his victory to come while a crowd cheers. Then he is crushed by an opening door of the ship transporting his opponent who steps off the transport and asks where his opponent is.