STORY 09 - Public Face
Meet with Michael in the sewers
As you approach the marked area in what looks like an abandoned construction site, you'll find three toads standing guard. To bring them down, you can either rush each one with your machete (or weapon of choice) flailing madly. Players with camouflage can take down a biter outside the site and then take down the unsuspecting toads. Or you could just pull out a gun and dodge around while you fire at their heads.
After choosing and executing the approach you desire (I personally went for the wildly flailing machete method), clear out the remaining couple of hazmat suit wearing biters. Preferably without detonating them so you can avoid the summon viral trigger. Once the site is clear, look for the marked sewer entrance in the middle of the area and descend into the depths once more.
Paddle down the underwater tunnel and break the surface in the first room to find a way out above the water in the South wall. Open the marked door to enter the sewers proper.
As you enter the sewers, you'll spot a group of biters in the tunnel ahead. You can ignore them if you want and just drop into the water in the gap in the grate in front of you. Swim along the waterway and emerge on the other side of the metal fence. Follow the only path you can take here through the pipe and fall down the vertical drop into the water below.
Swim back to the surface and climb out into a new pipe. Be careful as you approach the next room as a bomber will likely be present (use ranged weapons to detonate it if it moves towards you). More biters will be clustered in the room as well. The funniest way to deal with them is to lure them into your pipe and then use the grapple ability to throw them down into the water, killing them instantly. Funny but slow.
Use your preferred normal approach to clear the room of biters. Travel along the marked pipe filled with flares and climb out to the floor above at the far end. Open the decrepit wooden door here and then the next one to finally meet up with Michael.
Take the explosives
More explosives huh? Crane doesn't have the best track record with these things (or at least, keeping people alive while he plants them). Grab them off the cupboard behind Michael.
Reach the 10th floor
Unbolt the red door next to Michael and ascend the stairs behind it. The door to the ground floor (floor 00) is locked, so keep traveling upwards to the first floor. The floor is eerily lacking in enemies. Move to the far end of the floor and use the elevator call button to open the strangely still functional elevator.
Step inside and ride the elevator up... Thunk! Or not. The elevator will stop at the fifth floor, so you will need to climb out of a hole in the roof of the lift. Use the black and yellow painted ledges here to climb up the shaft and you'll come out on the 9th floor. Only one more floor to your objective.
Enter the apartment at the far end of the floor (where the marker pointing up is) and clamber up the large hole in the roof to arrive on floor 10. There's a pistol at the bottom of the hole you just climbed up as well if you need one.
Plant explosives in apartments 105 and 106
Loot your way through the first apartment before moving out into the corridor and engaging the biters within. Clear the biters as you go and enter the apartment at the far end of the corridor. You'll want to be very cautious here as a bomber may be lurking in one of the rooms, along with a handful of biters.
Work your way through the apartment to reach the other side of the floor and another corridor full of biters (watch the map as you battle as a viral may crawl out of a hole in the wall of the corridor and attack you from behind). Once the corridor is clear, carefully enter the first apartment on the right (106) and take out the toad and biter hiding within.
Enter the apartment's kitchen and interact with the stove to plant the first explosive. Once done, retreat back to the corridor and enter number 105, the next apartment along. A crawling and regular biter should be lurking inside. Kick them to death or just bludgeon them and trigger the stove-top to plant the second explosive.
Plant explosives in apartments 134 and 137
Sounds like Michael's having problems. Thankfully it's not your problem. Head back to the other side of the floor, through the apartment you traveled through earlier and ascend the stairs to reach the 12th floor. As you get to the top of the stairs, a viral will burst through the door for you to deal with, followed by two more of it's chums and biters.
A bomber will be surrounded by biters in the corridor through the doorway, so use a ranged weapon or retreat back down the stairs to detonate it. Once it is gone, you can freely deal with the remaining biters.
Move down the corridor, cleaning up any remaining biters along the way. When you get to the open double wooden doors, round the corner carefully as there is another bomber waiting for you. Once it's popped, turn around and look for a hole in the ceiling near the wooden doors. Climb up here to finally reach the 13th floor.
A viral will likely greet you as you arrive, so cut it down before moving along the hall. Clear out the biters from the hall first before going into any apartment to make life easier later. Apartment 137 will have a bomber waiting just inside the partially ajar door. If you are skilled enough, you can lure it into detonating against the door.
When it does, a viral will appear for you to deal with. Do so and enter apartment 137. The bomber should have cleared out any enemies inside, but if any remain cut them down. Flick on the gas and drop the next explosive before returning to the hallway.
Walk down the corridor to the apartment at the end (134) and open the door for a scene. Here you'll stumble across your first shrieker. These infected children emit a horrendous din, forcing you to cover your ears when in close proximity. This is painful in two ways, it prevents you from using melee weapons against them and also attracts any infected in the area.
When you regain control after the scene, you'll want to back up and deal with the virals first to avoid being swarmed. Keep moving as more biters keep arriving, rapidly filling the corridor. A shotgun or powerful melee weapon is your best bet here as noise isn't really a concern as the shrieker is making a racket anyway. Toads will also be spitting for acid damage, so take them out immediately after dealing with the virals.
Retreat and kill the infected as quick as you can to eventually bring the hall back to some semblance of sanity. Pull out a pistol or rifle when you get back to apartment 134 and blast the shrieker to hell. Now you can enter, head for the stove and place the last explosive.
Find Michael's detonator
Descend back through the apartment complex the way you came to get back to floor 00. When you show up, you'll run into a goon and mob of biters. A good tactic is to make a run for the staircase. This stops the goon slamming seven shades of shit out of you as you can back up the steps while you deal with the biters.
Once they are all dealt with, descend to level -1, the basement. A lone flare will mark the location of the detonator.
Leave the building and detonate the charges
After picking it up, drop down the nearby marked hole and travel along the pipe and through the open gate ahead. Follow the trail of flares and arrows until you enter a trench leading to a small area with a goon inside. Stay in the trench and take potshots at it's head with a pistol or rifle to bring it down safely. If it does come down the trench, you can climb to the level above and shoot it from above the trench.
Once the goon is no more, climb the ladder in the small room he was in and use your parkour skills to get across the catwalks to the open pipe above. Follow the linear path from here to finally arrive at the red door leading out of the sewers.
Detonate the charges to let the world know there are people in the quarantine
Open the door to trigger the scene and detonate the explosives. After watching the display, Troy will talk to you. Keep in mind you still have control at this point and are likely surrounded by infected, so start moving. Oh, and the mission's over by the way.