The literal term "fallen angel" appears neither in the Bible nor in other Abrahamic scriptures, but is used to describe angels cast out of heaven or angels who sinned. Such angels often tempt humans to sin.
Cast out of heaven = The Red Line
Sinned = Wiping out his own race
Lunarians were the true angels of One Piece world (Gods) not the Skypieans. King sinned against his own race and his wings turned black like the ones of a Fallen Angel (Demon)
Shockwave attacks are ultimately. more affective against durable opponent than blunt attack that basic logic and if merely blunt attack hurts you then a Shockwave one will be devastating.. recall Lucci's Rokogun