Convenient Weakness Placement

Sometimes, a villain's weakness is conveniently placed near him for no logical reason. For instance, you've tried shooting him. You've tried blowing him up. You've tried hitting him with a frying pan. You've tried everything short of throwing him into a boiling lake of lava, and he still won't die. But wait...an unlikely and specific series of events just happens to have landed you in close proximity to exactly what you need—often lava. (Obviously, lava is the panacea against an otherwise invincible villain.)

This is made all the more peculiar by the fact that the villain's lair is often built directly above lava, the one thing that can destroy him. Supervillain Lairs in general tend to be built conveniently near environmental hazards which may sometimes become active when the villain is defeated and spare the hero the need to kill him. This also helps the hero come out of the fight without killing anything. But, obviously, this trope doesn't just apply to lava.

In Video Games, it's very much an illustration of the Rule of Fun and one of the Acceptable Breaks From Reality. (No one wants a boss fight to become unwinnable when you've run out of ammo.)

This is sort of a combination of Benevolent Architecture and Contrived Coincidence.

Several subtropes exist for video games. See Boss Arena Idiocy, Puzzle Boss, Ring Out Boss, Bullfight Boss.

Anime & Manga

 * In the Hueco Mundo arc of Bleach, Las Noches has an artificial sun mounted on the ceiling, the only source of sunlight in Hueco Mundo. Like the real sun, it negates Aaroniero Arruruerie's shapeshifting powers. Aaroniero has no idea why his boss decided to do this, but at least he provided a nice, big room with ample shade.

Film

 * Terminator 2: They threw the T-1000 through a wall, they shot him in the face with a shotgun, they blew up a huge truck with him in it, they froze him with liquid nitrogen and broke his frozen form to pieces. The latter method seems to have worked, but they just happened to be right next to a steel forgery/construction site with a boiling pool of "lava", which thaws the pieces, which reform. They eventually force him into the lava and it STILL takes him almost a minute to die.
 * The Wizard of Oz: If water causes the witch to melt, then why does she have a bucket of it in her lair?
 * She still needs to fight fires? This makes sense, considering her own Playing with Fire abilities, but she should really have chosen a different location to set Scarecrow on fire.
 * In the original book, the witch kept Dorothy as a drudge and servant, and it was a bucket of mop water Dorothy was using to clean the floors when she finally snapped.

Literature

 * This is lampshaded in Carpe Jugulum, when they discuss how proper vampires are supposed to keep their castles filled with picture windows that can be opened dramatically and ornaments that can be bent into holy symbols. The reason being, if you make it easy enough to kill you off temporarily, no-one will bother trying to kill you off permanently.

Video Games

 * Resident Evil 5: Wesker has proven himself to be the most nonchalantly epic villain ever. Chris and Sheva have shot him, blown up a missile in his face, and injected him with his own poison. After all that, he mutates into a huge monster. method to kill him is forcing him into lava. But the plane they were on, which is unmanned at this point, coincidentally lands INSIDE a volcano, where the monster is forced into the lava. Even after that, he still survives for quite a while in the lava, and is only finally taken out by ballistic missiles.
 * More than a few Zelda bosses are only harmable by something in their lair.
 * One Dangerously Genre Savvy boss in The Legend of Zelda Spirit Tracks actually destroys the feature you're using to harm him.
 * An arguably-justified time that it happens is with the Bomb Flowers in The Legend of Zelda Ocarina of Time's Great Dodongo's Lair. Bomb Flowers apparently thrive in hot climates, and can be equated to a dangerously-explosive weed, processed versions of which are the Gorons' major export, and the 'normal' bombs you use through the majority of the game.
 * Any Bullfight Boss in which you have to make the boss charge into some damaging, natural feature of the arena. (Like a charged Tesla Coil when the boss is only vulnerable to being electrocuted.)
 * Melchiah from Soul Reaver. The only way to damage him is with three one-shot devices in the room. And the last of the three just happens to be a meat-and-bone-grinding death mechanism in the exact center of the room.
 * Half-Life 2: Episode 2 features an example during the fight against the Attack Choppers. Their weaknesses aren't NEAR them...they're IN them. The bombs they drop can be shot back at them via the Gravity Gun!
 * In the Pokemon games the Pokemon needed to counter the gym in any given town are usually found outside. the most Egregious examples of this are in Black/White where the first two gyms have people standing outside who specifically tell you where to go to get the Pokemon you need to use.
 * Super Metroid: The Crocomire fight. It's invincible and hitting it with missiles only pushes it back, toward a lava pit which instantly kills it.
 * The NES version of A Boy and His Blob. The evil king can't be harmed by anything except vitamins. Guess what he has a jar full of, sitting precariously on a ledge right above him?
 * Portal 2 lampshades this, momentarily averts it, and then plays this completely straight.  says this to Chell at the start of the Final Battle: "Four-part plan is this: One, no portal surfaces. Two, start the neurotoxin immediately. Three, bomb-proof shields for me, leading directly into number four: bombs. For throwing at you." Or, everything you used to take out GLaDOS in the first game. It looks for a moment like this will be averted... and then the player notices   However   is an idiot.
 * Then of course there's the great big button you have to hit to finish the fight, sitting there, plain as day. Just like earlier. Just have to mosey on over and push it, right?  Probably was about the smartest thing  did in the entire game; not his fault it wasn't instant-death.
 * In Chip 'n Dale Rescue Rangers, the unbreakable red ball conveniently present in the Boss Room is the only thing Chip and Dale can use to damage bosses.
 * In Demon's Souls, the area where the Storm King is fought is also the resting place of the Storm Ruler, a sword with a Razor Wind power that only works in that area which is perfect for chopping up the overgrown flying fish.
 * In The Godfather 2, it's always a good idea to take the fight to enemy Made Men. Their hangouts usually contain what is needed to achieve their kill condition. For example, an early one will only be Killed Off for Real if he takes a dunking. So where do you find him? Next to a river.
 * In Super Mario Bros. 2, the main villain Wart is described as having an intense hatred for vegetables. Thus, vegetables are the only things that can damage him in the final battle. Cue there being a vegetable-making machine right in the room with him.

Western Animation

 * Codename: Kids Next Door: In "Operation: I.S.C.R.E.A.M.", the Delightful Children used the equipment in an ice cream factory (ice cream men are mooks for the villains on this show) to create  a fifty-foot-tall monster made of ice cream that was nearly unstoppable. The heroes were saved when Numbuh Three turned on the factory's heater to maximum and it melted. Why did someone put a heater in an ice cream factory, of all places? In the villains' defense, even they didn't know.