So bear with me, but I want to try a puzzle-solving mission instead of a combat heavy mission. I am not sure that this is the best medium to do it with, but here goes. You need to:
1.) Find a way to go from duct exit to duct entrance by crossing a large corridor (sewer waterway with platforms on either side), either by scaling the ceiling, firing a rope or other such device across, or climbing down one wall to creep across the room and then climb up the far wall and into a duct. All the while avoiding being noticed by guards and pedestrian foot traffic.
2.) Safely descend down a slippery chute and gently entering the surging water of another pipe, rather than falling into the water and being swept away.
3.) Coming up for air under grates spaced in regular intervals as you traverse the final water pipe in order to climb up into the old meeting hall and hand over the summons to the mobster leader.
I envision this to be a freeform puzzle where I explain the obstacles you face and you plan on how to overcome them. Your characters will spend the evening and the next day preparing. Then we'll execute the plan and make any necessary skill, attribute, and saving throw checks along the way.
You should discuss your plan and ask your questions here, and then we'll move back into the IC thread when everyone is ready.