You must defeat Deathstroke after reaching the final phase of the Campaign for Disarmament side mission. What's interesting, it's not a duel during which you will use melee weapons since both Batman and Deathstroke will be using vehicles during the whole battle.
After parking the Batmobile in the marked location the battles will start. At first you should completely ignore the machine controlled by Deathstroke. Instead, focus on destroying the five Cobra tanks accompanying it. As you probably remember from the main storyline, you must carefully drive to those tanks from the back (important - they scan the territory around them from time to time!) and wait until they are targeted. A single hit should be enough to destroy a tank. Remember to get away from the place of attack immediately after shooting. If you get targeted by a Cobra, you must hide in a back alley as quickly as possible (afterburner can be helpful).
Focus on Deathstroke's tank once all Cobra tanks are destroyed. In this case you don't need to aim at any armor's weakpoints. Once you get close to enemy machine, start the regular attack with 60mm cannon. During the assault try to avoid taking any damage from retaliating enemy. Use dodge steering engines to get away from red beams and circles appearing on the ground. Additionally, use Vulcan cannon to shoot down rockets flying at Batmobile. Deathstroke's machine isn't extremely resistant and destroying it should take around half a minute. Deathstroke will be automatically captured by Batman - you don't need to help him in any way.