Expert No Mercy is a major pain. I've had to redo the finale in so many different ways until we finally listened to the smart guy and camped the stairs in the building. Two downstairs, two upstairs. Common Infected had no chance, Special Infected hardly touched us, and we dealt with the Tanks fairly well. Hell, even the door we were shooting through wasn't blown off its hinges by the time the helicopter came.
Maybe we were just lucky though. I'd like to try the stairs again to see.