Well I'm hardly an expert, but that's the easiest finale I know of, unless Valve fixes it. There's one spot on a rock where you can crouch-jump up. All four survivors can get up on the boulder, and just shoot/melee zombies in front. If you're careful you can pretty much take zero damage. When tanks come, you can throw a molatov and be shooting them from a distance, and close-up shotguns should finish them off...if you're unlucky you might get hit with a rock, though I heard autoshotguns may even break rocks midair.
All you really have to worry about is the smoker which will periodically appear and go to your far right, out in the deep water. A sniper watching that area can easily kill him though.
On the No Mercy finale, you can hide in the inside stairs, outside stairs, or under the ramp, and either autoshotgun the tank (not sure how well that works on expert) or just run around and let him burn to death.
On the Dead Air finale, I had a tough time...my team ended up defending in the spawn closet at the very back. Just melee/shoot and run out when the tank comes to the open area. Do NOT get near the luggage carts etc., since the tank will send them flying into you.
Blood Harvest finale, I dunno, that's the last campaign I need to get expert on.