Darkness is an Eldritch Abomination (or some form of god). He cannot be beaten by conventional means.
Darkness is powered by an Eldritch Abomination or some form of god. It makes sense that he'll take a while to be beaten while proving a constant pain in the arse.
Alternately: The Weaksauce Weakness was a lie Darkness fabricated to find his enemies, who get curb stomped when they try to use it on him.
Parodied: Darkness is absurdly powerful but he's also a dunce.
Deconstructed: His Mary Sueness is provided by someone more powerful than him; once he loses his master's favour, Darkness is stripped of his power and can finally be thrashed.
Zig Zagged: Darkness the King of Dark is insanely powerful, trashing all who oppose him. But in a straight-up fight with the heroes, he gets completely thrashed by The Power of Friendship, and his secret benefactor strips him of all his lent powers in disgust. Busted down to lowly Mook status, he promptly makes a meteoric rise through the ranks on his own strength, earning back his gifted powers... just in time to get slammed back down by the heroes in a Curb-Stomp Battle.
Averted: Darkness appears to be tough but he's defeated for real on his first appearance.
Enforced: The writers want an absurdly powerful villain to build up anticipation, and sales, for his inevitable defeat.
Lampshaded: "Isn't there anything you can't do, Darkness?!" "Yeah, I cannot be humble about it, pathetic fool!"
