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The first stage of the No Mercy campaign takes place in the apartments. This level is actually shorter than it looks, so don't allow yourself to be overwhelmed. You will start out on a rooftop - after grabbing weapons and health kits, you have two options: you can take the stairs, which is safer, or you can shoot out the nearby skylight and drop into a bedroom. The second way is a little quicker and can surprise the infected if they're not ready, but it can also leave you exposed and separated, so be careful. Move quickly through the apartments, there aren't a lot of opportunities for special infected to jump you. Check closets and bathrooms for molotovs, pills and other goodies. When you drop through the hole to the bottom floor, make sure you and your allies go all at once - otherwise a crafty hunter or smoker could take advantage of the situation.
Get to the alley quickly - once you get there, keep your eyes on the rooftops. Smokers will try and grab you, and hunters and boomers will try and get the drop on you from there. Almost immediately to your left while moving through the alley is a room with shelves and boxes - check here for molotovs and pipe bombs. Once you reach the end of the alley, go immediately right. There is a door here that lead into a building. The first door on your left opens into a room that will always have a bottle of pills and might have a healthpack - don't miss it! Continue through the building - it is less exposed inside, and can get you close to the safe room. The back room of the building is dark, but look around, there are usually molotovs or pipe bombs. From there, you can see the subway entrance. Move as a group, and don't hit the car! The alarm will call the swarm. Watch for smokers who will try and pull you toward it. Make it past this spot and it's smooth sailing.
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This level is much more difficult than the first one, so be on your toes. As soon as you exit the safe room you have two options - you can proceed directly down the stairs or you can go down the tunnel to the right. This will take you to an air vent. It's a little slower, but safer, and you might find a molotov or pipe bomb along the way. Follow the stairs down to the turnstyles. You may find your first weapon upgrade here - if not, it will likely be a little farther in the level. Search the booths for goodies. Head up the escalators, then turn left and head down the tunnel. Check the train cars for molotovs, and be careful around corners. You should reach a table pretty soon which will either have ammo or guns - arm yourself or reload. You may encounter your first tank here, so be careful. Once you pass through the final train car, look right to see if there are any consumables in the room with the pipes - you may also find a health pack or two on the table by the door. Head up the stairs, but wait before entering the room with the pillars. Check for special infected and throw a pipe bomb if you have one.Clear the room and head up the stairs.
Once you're in the room with the generators, grab a molotov off the table by the door if you don't have one already. Check the room by the chaingun for medkits and pills, but watch the hole - smokers will try to pull you through. When you activate the door, the horde will come, but don't panic! Find a corner and stack up. Underneath the pipes on the wall by the door is a good place. Once the door is open, head up the stairs and into the room. Close the doors. There should be guns and ammo here, so grab some. Enter the rest of the building, and keep your eyes peeled - these rooms will almost always have goodies. You can either shoot out the windows and drop down, or take the stairs to the street. Either way, move quickly once you're there, you're easy prey. Avoid tripping the car alarm in front of the safe room door and you should be safe.
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There are two ways you can go out of your safe room in this level, but you most likely want to go directly ahead, into another room with boxes and shelves. Be aware of the rooftops as you go, you may get jumped. Check the room for molotovs, then move ahead. To your right is a lit garage - there may be weapons, goodies or healthkits here, so look around. Once you've got everything go straight down the alley, then left - keep your eyes open for items on the ground and infected on the roofs. On your right will be a door to a kitchen, enter it. The entrance to the restaurant is glass, so take this opportunity to shoot any unsuspecting infected you see. You will be in a truck dock of sorts - at the end of it is a gas station. You should shoot this now - it will draw attention to you, but if you don't, it could go up by accident and can be a real problem. What's more, it opens up an extra room which you can access via the debris - check there for weapons and items. When you're ready, activate the lift. You don't have to be on it, it has a ladder, but it's a good idea, since you want to move quickly here. Watch for smokers who will try to pull you off as you make your way along the roof. Once you reach the room at the end of the path, make sure you're all together before you drop into the hole. This room will have healthpacks and ammo, at the very least. Don't stay here for long though - infected will come after you, and smokers can pull you through the hole. You can also open the door to the dock from here if you left someone behind. As you move down the hallway, check for items in the rooms. Run quickly through the room with the boxes, down the hallway and into the stairwell.
You should be near a room full of machinery. The entrance to the sewers is near. There is one manhole in the middle of the room, and one in a room off to the side. The one to the side is safer, but the one in the middle is closer to your destination. Assuming you took the safe route, go down the sewer to the first left and follow it to the second tunnel entrance. This should be right below the other entrance. Follow this route - it will take you directly to a room with weapons and items. Be aware, this is another likely tank location. Follow another tunnel from this room. Upon exiting, the ladder to the surface will be to your right. This can be a difficult spot - special infected will likely be waiting for you at the top. Go all at once, and be alert. If you survive, the entrance to the hospital is right there.
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You're almost there! But first you've got to get through Mercy Hospital itself, which has been overrun. As soon as you exit the safe room, the infected will try and hit you. Watch out for the the boomer. Head quickly to the end of the hall and into the stairwell. Shut the door behind you. Head through the hallway and the open room, but watch out for hunters and smokers - it is a little exposed. There is not a whole lot be found in this part of the level, so put the speed on. Head up the stairs, down the hall and into another stairwell. These are safe places, so close the doors and heal up if you need to. Once you're on the third level, you want to really book it. Go down the hall, through the double doors, and then right past the barricade. Keep your eyes peeled for pills and healthkits, but get to the elevator as fast as possible. Grab some ammo, get into a safe spot, and start the elevator. If you stack up behind the gurney, you are almost completely safe from attack.
Fend off the swarm and jump in the elevator fast. This is one of the safest places in the game, so take the time to heal if you need to. Once you're at the top, you will almost definitely meet a tank (unless you fought one already). The vents behind the elevator are a safe place to lie in wait for it. Once it's dead, you're almost home free. Just watch out for smokers when you're close to the edge. Follow the path (it can be a little confusing, keep your head) and you should reach the safe room safe and sound.
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Here it is! All you have to do now is wait for the chopper to come. This is harder than it sounds. Make sure you grab a healthkit and an explosive from the table. Move quickly through the hospital section, you probably won't encounter many infected here. There is usually a bottle of pills in one of the rooms - look for it, but don't take too long. Go through the elevator shaft until you reach the ladder to the roof. The special infected will probably be waiting for you - toss a molotov onto the roof and you might get lucky. Go up all at once and be ready. There are probably going to be a whole lot of infected here, but don't panic. Kill the ones in your way and head to the end of the roof where the chaingun is. There are supplies and ammo here, so fill up. When everyone is ready, activate the radio twice. If you trust your team, have someone on the chaingun, otherwise take up positions against the wall. You will have to kill a lot of infected. Watch the edge of the roof, smokers will try to pull you down. You will fight two tanks. There will probably be molotovs near your position across one of the pipes. Use it one the tank as it comes up from below. After you kill the second tank, that is your sign to head to the helicopter. Hurry, because if you don't, a third tank will spawn. Once you're in the chopper, the game is done. Congratulations! You survived. For now...
Be sure to check out some of our other great Left 4 Dead articles, like the Left 4 Dead: Infected Strategy Guide, and the upcoming guides to the other Left 4 Dead campaigns.