DISCLAIMER: I do not own DC Comics, the Justice League, Batman or Assassin's Creed.
The streets of fourteenth century Italy were buzzing with life. The dark night streets were filled with people going about their affairs. Batman stood at the top of a building, looking down at the people, completely invisible to them. Batman had come in a time machine to this time because there was an event in history that needed to be changed. The legendary assassin known as Ezio Auditore Da Firenze had to be apprehended.
Batman saw Ezio, just where he was expected to be. Batman dropped down behind Ezio, but Ezio turned to face him, stealth proving ineffective against the assassin. Ezio readied his wrist blades, while Batman readied two batarangs. Batman suddenly flung the batarangs, but Ezio blocked the both of them, before drawing his crossbow and firing a bolt at Batman.
Batman caught the bolt in his hand, charging at Ezio. Ezio met Batman head on, and the two began exchanging blows, Ezio's blades ringing against Batman's batarangs and armour. Batman readied his Taser, and started to prepare to attack Ezio with it. He hoped modern technology would catch the fifteenth century assassin off guard, but his hoped were not to become reality.
As Batman attempted to use his Taser on Ezio, Ezio grappled Batman, attempted to stab him through the face with the wrist blades. Batman intercepted Ezio's arm, twisting into hyperextension, and then slammed his foot into the assassin's instep, before finally punching him backwards and then kicking him onto his back. Batman then tossed a flash bang grenade onto Ezio's chest, before diving away as it went off.
Batman stood again to see that Ezio was still standing, his eyes dazzled, but he had avoided the direct blast of the flash bang, so he was not stunned. Ezio raised his forearm and fired a shot at Batman, but Batman's armour protected him from the old, almost primitive bullet. Batman then slid along the ground, kicking Ezio onto the ground with him, before punching Ezio unconscious.
Batman stood up. "You couldn't just come quietly, could you? You'll be released once we have what we want. Once the Apple is destroyed, and once the Templars are arrested, and the assassins made to retire."
Reasons for this outcome: 1. While Ezio is a masterful assassin second only to Altair, Batman is a master of hundreds of fighting styles, and regularly faces enemies far deadlier than anything Ezio has faced. 2. Ezio wields many weapons, but they are limited by their age, while Batman's gadgets are state of the art, and he wields weapons Ezio has never seen before. He also wears far superior armour.