In Supergirl fanfic Hellsister Trilogy, Brainiac is unceremoniously and unexpectedly blasted in half from behind by Darkseid, who after barging into the gathering of villains and doing away with the Coluan proclaims himself leader.
Young Justice: Darkness Falls has the death of Vandal Savage. The original founder of the light, and basically the defacto leader of them, he gets an omega beam to the back just because he got greedy and killed Superman rather than beat him down. Then again, considering that he went against the will of Darkseid, there isn't much else one could expect for this action.
The Naruto/One Piece crossover The Unchained has an elderly Madara ripped off of life-support in chapter one, leaving his least competent minion as the only one alive to carry on his legacy. Also, the story can't truly start until the Third Hokage is killed by a sleeper agent.
Samuel Campbell attacks Ruby "with the coordination of a sloppy drunk" and she just pushes him off the catwalk. The text even describes it as "the most anti climactic fight sequence in the history of fight sequences."
Shortly later, Ruby disables Crowley with a single (albeit incredibly karmic) spell and stabs him in the stomach. The author admits the anti-climatic end in an author's note, and says it was because the real important thing in the chapter is her death moments later.
Invoked literally in the two prologues to Tripocalypse, in which the Archangel Michael is incapacitated by dropping the deployable portion of an AVLB 104 combat bridge on his head. The Archangel is knocked out long enough to fall into the portal to Hell that he had created moments before.