Devilman's first battle. Frightened and maddened out of his wits due to being an human in an underground chamber filled with menacing, nightmarish monsters, Akira dashes off. Right then Amon tries to possess him... and Akira manages to defeat him -one of the most powerful demons ever- in his mind and steal his power. Straight after he transforms into Devilman and starts brutally slaying the several dozens of demons filling the room.
Even before that, Akira shows some much needed guts and 'pure of heart' when he stands up to three punks who killed a bunch of rabbits. He takes a lot of pain in just protecting the last rabbit and the gang boss backs off when he sees how tough he is.
In the OVA "Amon: Apocalypse of Devilman", Akira loses his human heart due to a tragic loss, and thus is fully taken over by Amon. In the climax, Amon attempts to destroy Akira in a battle within their conjoined soul. Akira proceeds to show just how badass a true Devilman is.
Akira is mentally and spiritually broken when Amon comes a-knockin', and for most of the fight, Amon completely dominates the battle. And then, Devilman gets a Heroic Second Wind. He has nothing left to fight for, but he gets up, and he pounds Amon in the face. Once. Twice. Repeatedly. Even though Devilman has lost everything else, he still overcomes Amon with the determination of his human spirit.