Go to the waypoint on the map.
Stand above the sewer opening, and use the appropriate buttons to open it.
Follow the path, when you face the Ninja(enemy that counters your counters), you need to use double counter(when 2 blue markers appear above him), then when you attack him, he will counter you and attack you, when he counter's your attack you need to counter that counter, it's a back and forth game.
Move forward through the sewers, you will need to use the Grapple Gun a couple of times to climb up, when you reach the button press it.
You will then be prompt with another Glide Attack, use the appropriate button to attack the enemy and move forward through the sewer hole.
You will face another group of enemies, with another Ninja among them, use the same technique like before to defeat him.
Go up using the elevator, then defeat the Tough Enemy with the Cape Stun like before.
Then go to the Server and press the appropriate button to download files from it.