Once in the sewers, just follow the path you're on until you come to big gap. Drop down and take out the thugs here but be careful because one of them is armored and there isn't a whole lot of room to maneuver. Make sure to use your evade to jump over enemies so you can get some space. Once you get rid of them, head down the tunnel and take a right. Crouch under the gate and then pick the line launcher. You want to aim and shoot to go across the water but you want to release your aim and then re-aim as you get to the intersection. You want to aim left and then shoot again. This will send you in a 90 degree angle. Land and then take a left. Go down the hallway and take a right so you're standing on wooden boards above a group of thugs. Use a takedown to go through the wood.
This fight is a little complex. Thankfully there are no armored enemies but there are weapon crates scattered around the map and milk crates that enemies will try to throw at you. That can create quite a handful for this fight. Your priority should always be anybody going for a weapon. Take them out first with gadgets if you need to. After them, as long as you keep moving, you can skip the guys with the crates otherwise you want to go for them next.
Once they are cleared out, there is a panel on the Southeast wall. Hack it. Go through the gate you just opened and you'll find yourself in a familiar place. This is where the last jammer was. Near the jammer, in the northeast corner of the room is a door so you can continue your search for the assassin. Go through the door, use the line launcher to cross the water and then grapple up to the ledge above. Go through the door at the top and walk forward.
