Now, I know what you're thinking. "This guy is crazy! Does he honestly believe that one man can take on the Avengers?" "Batman is so over-rated." etc. etc. etc. Well, you're all wrong. Batman is not over-rated. He's just smart enough to defeat every single member of the Avengers. And I'm going to tell you how he would do it.
Let's start with a little set-up here. First off, I'm taking Batman as he would appear in the New 52, and for the Avengers, I'm using the characters from the upcoming film Age of Ultron. So, here's the line up.
Marvel: Iron Man, Captain America, Black Widow, Hawkeye, Thor, Hulk, Vision, Scarlet Witch, Quicksilver, Falcon, War Machine
Now, I know that some of those aren't likely starting as heroes, and not all play a large part in the film, but, for discussion's sake, this is the roster. So, it's only 1 to 11. Not terribly huge odds. Before we begin with the tail-whipping of the Avengers, let's go back and look at the time that Batman basically defeated the entire Justice League.
In JLA: Tower of Babel, Batman created plans to immobilize the entire JLA. These plans were stolen by Ra's al Ghul, to devastating effect. Martian Manhunter was covered in nanites, causing his skin to burst into flame, which was his weakness. Plastic Man was frozen and shattered. Aquaman was hit with fear toxin, and made afraid of water. Green Lantern was hypnotized by his own power ring. Wonder Woman was placed into a virtual reality with an unstoppable enemy. She was basically fighting herself to death. Flash was taken out by a "vibra-bullet" that gave him seizures. And Superman was exposed to red Kryptonite, which turned his skin transparent. In the end, Batman was able to reverse the attacks, but, it was clear that Batman could take out every single Justice Leaguer, with ease.
Now, let's apply this to our situation. We have to begin by assuming that Batman would have time to plan and prepare an attack on the Avengers. Here's how he'll do it.
Iron Man: Before we even begin, I'm just not delving into the whole "money is their superpower" junk. Money would have nothing to do with this fight. I feel like Batman would go for the heads of the group first. Now, an EMP gun wouldn't cut it here. Tony just has too many suits. And, assuming that Batman doesn't want to kill them, he wouldn't want to knock Stark out of the sky. A virus in JARVIS would be more effective. It would render all of the suits inoperative, and keep Tony tied down for quite a while. It would have to be a virus that completely changed itself every so often, or something that mutated like a real disease. Lacing his alcohol with a poison of some kind would work to remove Stark completely.
Captain America: Perhaps more than any other, this would be just a straight knock-down drag-out fight. Captain America has superior strength (which honestly is a touch debatable, since in some comics, Batman can bench press a thousand pounds.), and is no slouch when it comes to fight techniques, but Batman is a master of almost every martial art in the world. Captain is also not immune to poisons, or toxins. For the sake of argument, Batman would use a fear toxin on Captain, perhaps to turn him on the other, more human elements of the Avengers.
Thor: This guy is going to be tough. Thor has almost no weakness as long as his hammer is in his grasp. The trick would be to break his heart. Batman would need to threaten Thor's humanity. Perhaps trap Jane, or someone else he cared about. Perhaps an even better plan was what was used against Wonder Woman. Attack his mind, and let him fight invisible monsters till he collapses. If that doesn't work, well...bye-bye Batman.
Hulk: Been there, done that. In a cross-over, Batman beat the Hulk in hand to hand combat. He simply kicked Hulk in the solar plexus, causing Hulk to breath in. Batman had simply released a toxic gas into the air, and forced Hulk to breath it. It could work. However, a better way to keep Banner out of the way would be to place him in a situation where he can't Hulk out. Perhaps trap him in an orphanage, or a mall. Some place where there are many innocent people. Bruce's humanity would keep him from hulking, and render him useless.
Black Widow/Hawkeye/Falcon: Now, supposing that the fear-induced Rogers doesn't remove these two, Batman would simply use his superior fighting skills and tech to take these three out. Again, Batman relies on fear, and would likely pit them against each other, or scare them, and then remove them. Again, forcing Rogers to fight them would be scary enough.
War Machine/Vision: I know, I know. Vision is crazy over-powered, and War Machine is just a dude in a suit (A very boss dude, mind you, but...). Question though...what are these two connected to? Iron Man, and therefore, JARVIS. If Batman had already removed the AI from the picture, then both the android and the Machine would be out of commission. End of the line for them.
Quicksilver: Again, pull the same move that was used on the Flash. Now...the Flash can move so quickly that he can make time reverse. Quicksilver has only ever been clocked at a top speed of Mach 5, so...really no contest as to speed. All Batman would need to do is hit Quicksilver in the base of the neck with a vibrating bullet, that would disrupt his nervous system.
Scarlet Witch: Honestly, she may be the toughest heroine that Batman would face. Her psionic powers and the ability to manipulate probability (and ultimately reality) are some of the most powerful abilities in the Marvel universe. Again, Batman would need to rely on stealth, speed, and fear to defeat her. In this case, taking Wanda back to her childhood would certainly break her down emotionally. Taking out Vision would be a blow to her as well. Another way to take her out would be this: trap her in her own mind. Use the nanites to create a reality around her that fed off her own thoughts. She is one hero who's mental health is often in question. Batman could easily break her mind, and ruin her.
Now, these are pretty vague plans of destruction and ruin. Here's how I see it all going down. Batman starts with a virus in the JARVIS AI, rendering Stark, Rhodey, and Vision inoperable. He then hits Rogers with the fear toxin, and Steve goes to work on the SHIELD agents in the team, and anyone else. Batman then uses the nerve gas to knock the Hulk out, then traps him a crowded area, to prevent him from Hulking out. Then he hits Quicksilver with the vibra-bullet, causing the poor boy to fall into uncontrollable seizures. He then implants the nanites in Thor and Scarlet Witch's minds, causing them to fall into the fantasy worlds aforementioned. By this point, only one or two of the heroes would still be in fighting form. Batman could easily dispatch the remnant. And, in one day, Batman has defeated the Avengers.
Now, if we're talking movie franchises, then...the Avengers win every day of the week. However, if we are talking characters, then Batman would always be three steps ahead of the team, and would always be prepared to remove them. I really hope that you've enjoyed this slightly ridiculous mash-up, and I hope that you'll discuss it in the comments.