For a finale I suggest using something along the lines of the Crash Course finale where you have to run out and activate some sort of switch (a generator in CC) and have it in a spot with easy ambush points and a long way from any defendable spots (like you would need 2 pipe bombs to stop the horde from touching you length). And for crescendos I suggest you don't make permanent gauntlets like in Dark Carnival 4 where the horde spawns right on top of you at the end (thats pretty cheap IMO at least)

Unfortunately valve took out the entity needed to make this kind of finale. I was trying to do something similar to this for my campaign only to be disappointed when I found out that it cannot be done in l4d2.