You begin the mission by pressing Circle to detonate the charges on the bridge, lowering it. You'll be thrown straight into the boss fight!
BOSS: GIANT KNIGHT
You'll be fighting this monstrosity with Sly. You need to use the trampolines to get on top of the towers, then shoot an arrow through the ring and hit the weak spot on the arms, then use the rope to get to the arm before it can cut it with a laser, where you need to shoot a couple of arrows with R1 until the armor falls off.
Of course, the boss won't just let this happen, shooting lasers that follow you as one of its attacks. It can also shoot missiles into the air, which are designated on the ground as targets. The boss also likes to send a shockwave on the ground by pounding its fist.
After you hit both arms, go onto the middle tower, where you will shoot a bomb into the weak spot there. You must do the same thing again, with the Giant Knight's attacks getting more frequent. Once you hit the middle spot again, the next phase of the battle begins.
It's a duel to the finish with between Bentley and the boss, in two mechanized suits. This is a pretty easy battle, as long as you don't charge in and try to rush. The Square button attacks, while X dodges. What you want to do is wait until the boss gets in close and whiffs on an attack, then move in quickly and unleash a three-hit combo of your own. Keep doing this until the boss is finished, with a QTE at the end.