Rooftop Finale

Upon reaching the roof, the Survivors find a makeshift defensive position in one of the main structures, and must use a radio to alert the helicopter to their presence. The pilot tells them to prepare themselves, and call again once they are ready. Once the second call is made, the helicopter will begin an approach, but the pilot warns that it will take some time to reach the hospital.

The Survivors must then hold out against waves of Infected which swarm their position, including several Special Infected and at least two Tanks. After fifteen minutes (with the pilot giving periodic updates), the helicopter finally arrives at the partly destroyed helipad, and the Survivors board it to make their escape. The Survivors may trigger an additional wave of zombies by shooting at the rotors of the helicopter as it lands.

It has been reported that if a Tank is allowed to reach the helicopter before it departs, it can wedge its arm inside, and actually prevent it from departing until it is "removed."

With all the Survivors aboard, the helicopter flies off, going behind the hospital before coming around and passing above the camera. The camera fades to black, and the credits roll.

The Original Ending
In the Director's Commentary, it is revealed that the original ending of No Mercy was that after they got on the helicopter, the pilot was complaining about how he was attacked while saving another group of people. After he died from the infection, the Helicopter crashed on the highway at the beginning of Death Toll. Valve thought that this left players with an unsatisfactory ending to the campaign.

High Roof Method

 * When you climb up the ladder and first emerge on the roof, do not leave the building that you are on. Instead, select one person to go down.
 * Do not call the radio yet. Instead, the person who jumped down should grab a first aid kit, and bring it back to the building. One at a time, each person stand on the very edge, and let the person heal them. Repeat this until the 3 left up top are healed.
 * The person who dropped down can now call the helicopter. The other three should stay on top, watching out for Smokers and Hunters.
 * Make sure you use the Minigun. Watch out for The Smoker as they can drag you from the high roof.

Radio Room Closet Method

 * Once you are on the rooftop and healed, get into the closet near the ammo and weapons cache.
 * Two people should be near the door crouched and meleeing, while the other two are behind them shooting, preferably with dual pistols to conserve ammo.
 * Once the helicopter is called, all four survivors should stay in the closet, until the first Tank appears. When the Tank appears, all four survivors should leave the closet, and defeat the Tank like normal.
 * When the Tank is dead, return to the closet, and repeat with the melee and shooting formation until the helicopter comes.

Helipad Method

 * Once everyone is well-stocked, go back to the ramp next to the helicopter platform. Go around to the base of the platform, and go under the ramp.
 * Crouch and back in as far as possible.
 * Have someone activate the radio, and then run back to you.
 * This area functions like the closet, except that you can shoot out the sides and top. (This functions as wall penetration, so damage will be reduced, but the smoker can't hit you)

Stairwell Method

 * Make sure everyone is well stocked with ammo and such.
 * In the stairwell in the radio room there are two doors, station two people at each door. One person to crouch and melee the other to shoot.
 * As long as they pay attention specials can't cause any problems.
 * When tanks come, scatter light on fire and play cat and mouse until its dead.
 * When Helicopter comes rush pad and exit. NOTE: It makes the finale easier if you place some gas tanks, propane tanks, etc around the pad so you can blow them as you leave taking some of the zombies out.

Glitch Ledge
On the highest area of the radio room, a survivor can get on a small ledge by slowly walking off the roof of the building. By using this area, none of the infected can reach the survivors except the Hunter and the Smoker, but they can be easily dealt with. By using this area, one can easily get the Untouchables achievement. This ledge was removed in the versus version, but a similar exploit exists on the high building to the right of the minigun when facing the helipad. Be advised that human tanks are more likely to toss rocks at you in this position.

"Jesus Room" Method
Note: This method is considered an exploit. When you first climb onto the roof and face the helipad below, you'll see a building to the right of the helipad. Go over to it, climb up the ladder and turn right. If you hop up onto this ledge while infected are trying to get you from below and crawl to the very edge, they infected won't be able to get up to you. If you position yourself directly above the door below, the infected will break it open and you can enter the room beyond. The door can also be opened via constant melee, as the door in The Sewers level, but takes considerably longer. This seems to be a room that infected spawn from during the finale, and if you hide at the very back of this room, no infected, special or otherwise, will be able to find you (unless they targeted you while you were entering or you're covered in boomer bile). If you're patient, swarms will eventually die and the tank will spawn. This is where it gets hairy. Sometimes the Tank dies on his own because he can't find you, but this doesn't seem to happen every time, so you may need to go out and kill him. Just remember that if you light him with a molotov and retreat back into this room, he WILL be able to find you. Once the second tank has died, you can run out to the helipad. With some luck, Untouchables can be attained with ease using this method.

Tank
If you corner a survivor on top of a roof, one good punch can send them flying off the top of the hospital to their doom.

Smoker instant kill
When a survivor enters onto the first part of the roof of mercy hospital, there is a smoker that is positioned on the rooftop to the left (only has a yellow air vent on it) that grabs the suvivor and pulls them off the roof. You will hear a loud crack, and this is assumed to be the survivors neck. It will instantly kill any survivor regardless on how much health they have. Positioning yourself as a smoker on the roof with the satellite will achieve similar results.