Basic Trope: Only The Hero can defeat the Big Bad; anyone else who tries will get owned.
Straight: Bob, thinking that Alice is dead, takes up her quest to defeat Lord Truculent. He manages to get past the malign troopers and Prince Scurrilous, and reach the throne room of the dark castle. Then Lord Truculent kills him with ease. At that moment Alice enters, and slays Lord Truculent.
Prince Scurrilous, thinking that Lord Truculent is dead, takes up his quest to defeat Alice. He manages to get past the anti-dark-lord army and Bob, and reach the Arcadia. Then Alice kills him with ease. At that moment Lord Truculent arrives, and slays Alice.
Alice and Bob reach the throne room of the dark castle together. Bob, aware of the prophecies, lets Alice rush into battle with Lord Truculent and just tries to avoid collateral damage. Suddenly, Alice falls dead to the floor. Desperately, Bob takes up her sword and continues the fight. After a few minutes, Lord Truculent is killed.
Subverted: Bob, thinking that Alice is dead, takes up her quest to defeat Lord Truculent. He manages to get past the malign troopers and Prince Scurrilous, and reach the throne room of the dark castle. There, he kills Lord Truculent. At that moment Alice enters, and gives Bob a congratulatory pat on the shoulder.
Double Subverted: Lord Truculent suddenly stands up and kills Bob before he has time to react. Alice then fights and defeats Lord Truculent.
Parodied: Alice and Bob reach the throne room of the dark castle together. Lord Truculent politely asks them to state their names and prophesied roles. "Bob, heroic sacrifice" says Bob, and explodes. "Alice, chosen one" says Alice. Lord Truculent nods and gives her a key-shaped sword. She stabs him through a keyhole-shaped hole in his armor, and he falls over dead.
Zig Zagged: The prophecy states that only Alice can kill Lord Truculent. However, Alice hasn't been seen near The Kingdom for decades, and the situation gets desperate as Lord Truculent quickly approaches.. Bob, who hasn't heard of the prophecy, attacks Lord Truculent, and kills him. Before he has time to celebrate this, the real Lord Truculent reveals that the dead one was a decoy, and kills Bob. Aerith studies the prophecy and decides that the specific statement of killing can be exploited as a loophole; As Lord Truculent reaches the city gate, she casts a paralyzing spell on him. He falls to the ground and lies still for a few seconds. Then he starts laughing, stands up and explains that the untranslated version of the prophecy uses a word that means "defeat" rather than "kill". He picks up Aerith by the neck and prepares to kill her, when Alice suddenly appears out of nowhere and starts fighting him. The battle is intense, and Alice inflicts several very serious wounds on Lord Truculent. However, to everyone's fear and surprise, she is very definitely killed dead by a mace blow to the head. Panicking, Aerith starts firing every offensive spell she can think of at Lord Truculent, hoping to buy the civilians some time to flee. Lord Truculent first seems unaffected by the spells, but then stops, wobbles a little and falls over dead... From one of the wounds Alice inflicted before dying.
The ridiculously overpowered anti-dark-lord army kills Lord Truculent.
Lampshaded: "Oh, hey Bob! How are you? You want to defeat me, huh? But you're, like, a supporting character. I think Alice would... Oh, nevermind. Go right ahead. Do you want a bigger sword or something? Should I fight with my left hand or something? Wait, give me a minute and I'll fetch my styrofoam mace."
Invoked: "Hello, Alice. I can understand why you take offense at my campaign of senseless murdering, and therefore want to stop me. However, shouldn't you let the anti-dark-lord army try first? I mean, they're professionals. Surely they have a better chance of success than you do?"
Exploited: The entire armies all line up, get ready...and then Alice calls a time out, asking to just get it over in a one on one duel. The big army fight is just a waste of resources.
Discussed: "Sir, I know that you assembled and trained the anti-dark-lord army just for a situation like this, but I assure you that sending Alice instead is a much better idea!"
Conversed: "Have you noticed how the outcome of these final battles are always determined by plot importance rather than skill?"
Deconstructed: Alice, knowing that the prophecy states that no one else can kill Lord Truculent, reasons that if she dies fighting him she is responsible for what would happen afterward. Therefore, she keeps postponing the eventual battle, and spends all her awake time training. Her childhood friend and only support Bob flees from the war-torn land and goes missing. Every day, civilians are killed by the thousands, as Alice never feels confident enough in her skills to seek out and fight Lord Truculent.
Reconstructed: Eventually, Lord Truculent seeks her out instead. She has no choice but to fight back, and thanks to her months of intensive training, she successfully defeats him. The malign troopers, now free from Lord Truculent's control spell, help the remaining civilians rebuild the destroyed cities. Alice, encouraged by her victory, embarks on an expedition together with the remnants of the anti-dark-lord army to locate Bob.