After completing ‘Occupy Gotham’ , ‘Armoured and Dangerous’ and ‘Own the Roads’ and sorting out the explosive situation from the first part of the ‘Campaign for Disarmament’ side-mission, you’ll get a radio message from Deathstroke who challenges you to take him on at Grand Avenue on Miagani Island.
When you are ready to fight, drive the Batmobile onto the island and when you arrive at the objective marker, the bridges will be raised and you won’t be able to lower them again until after you defeat the boss battle.
As soon as the fight begins, you’ll note the number of Cobra Drone Tanks on the island will instantly jump up to six. Five of these are your standard Cobra Drone Tanks, whilst the final one is Deathstroke’s tank of doom. To start with, you are going to want to attempt to isolate and eliminate each of the Cobra Drone Tanks before turning your focus to the main man himself.
Remember that you’ll need to hit the Cobra Tanks in the glowing section on their backs in order to destroy them and avoid their lock on attack as it can cause a silly amount of damage. There are plenty of side-alleys, ramps and tunnels to help you outmanoeuvre your opponents – just keep an eye on the mini-map to keep track of their locations and high tail it if you see yourself getting cornered.
Once you are finally down to facing Deathstroke, you’ll find that he is actually not that difficult, especially after having completed all of those Drone Tank defence sequences whilst hacking bombs. You can shoot him anywhere on his tank’s body to damage him, you’ll just need to avoid his attacks – he has an EMP, can shoot grenades similar to the Dragon Drones, lock-on missiles from the Mambas and has a Diamondback three pronged tank shell attack.
I found the easiest way to deal with him was to back away whilst kiting him around in a large circle and shooting him constantly with your main cannon when it recharges. This way you can see his attacks coming at you and you can make necessary adjustments to avoid them. The trickiest attack to avoid, as always, is the homing missiles (use machine guns to shoot them down) and being sure to dodge out of the way of the grenades if he throws more than one at once.
Once you have reduced Deathstroke’s health bar to zero a scene will play. Following the scene, drive back to the GCPD headquarters on Bleake Island and park in the indicated spot to complete the side-mission.