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 Expert having all gas cans needed: 15. 16 is also the maximum point limit in Scavenge Mode. 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 refuelling all in the one run.

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 characteristicly huge spoiler and blue paint.
 * there is a hidden jimmy gibbs jr. zombie