The Atrium

The Atrium Finale has the Survivors collecting gas cans to fill a Racing stock car in order to escape the mall in a similar manner to Scavenge Mode. After starting an elevator to get down to the car the survivors are immediately mobbed by hordes of infected while trying to get gas cans. The amount of gas cans needed depends on the difficulty. Easy and Normal being 8, Advanced is around 13, and on Expert 15 gas cans are needed. In single player mode, the non-player survivors will not attempt to fill the car up with gas due to the fact that they're protecting you from incoming hordes.

Strategies
One way to make things easier is to toss the cans over the rails next to the car. That way, you can fetch the more remote cans faster and also do the refueling all in the one run. Although this is risky, shooting a gas can may start a fire which spreads if the cans are too close. However, the gas cans respawn again in their original place.

In addition, work in pairs instead of as a group when throwing the cans down to spread out the horde and make the process quicker; the horde is focused on the center of mass of the survivors, and pairing up will spawn the horde farther away and in fewer numbers.

Trivia

 * This is currently the only campaign ending in L4D2 to have "finale" in the title, like the ones from the original L4D.
 * The stock car and its driver are likely references Dale Earnhardt Jr., a NASCAR Sprint Cup driver who drives the 88 car and Richard Petty who drove a Dodge Daytona and a Plymouth Superbird in 1969-70, respectively, in NASCAR with the characteristically huge spoiler and blue paint.
 * A cut Uncommon Infected was Jimmy Gibbs Jr. himself. He is said to have 1000 health and can only be manually spawned through the console, he is immune to fire and covers your screen like a Mudman. The Survivors also have special lines when seeing his zombie.
 * Oddly enough, when stepping out of the elevator, you are told "Rescue is Coming, Defend yourselves!" even though you're required to fill up your vehicle.
 * Left 4 Dead did this as well, when you refuel the plane in Dead Air during the finale, it says "Rescue is Coming, Defend yourselves!" even though it's sitting right in front of the fuel tanker.