I've written a short guide on how to get through the first part of 'Dark Carnival' on Expert Realism in my Games Computer Games blog.

Rather than re-create the entire thing here I'll summarize:-

1. You can get through Normal and Lower difficulties with little effort - but you need good tactics as well as being a good player to get through on Expert Realism.

2. You need to have places in mind where you should hole up, and stand your ground during hoard attacks, finding them when playing an easier difficulty level where you probably don't need them is probably easier than trying to find them when you're being pounded into the ground by infected and the only need 5 hits to kill you...

3. Know the levels, know the best route! It's possible to get to the Motel in Dark Carnival without going under the under-pass, and it's much easier.

More details are on the main blog.