My friend and I mostly only play expert realism. He's beaten all of the campaigns except The Passing, and I've "beaten" (just didn't survive) for Dead Center, Dark Carnival, and Hard Rain (sooo hard on ER). Actually, I think I only survived on Dead Center, since I've done that one a few times.
If you're having trouble with Dead Center on ER, you shouldn't be trying any other campaigns on ER. Dead Center is EASILY the easiest campaign for ER, nothing else comes close. It's kind of easy mode compared to the other campaigns. I think I've actually beaten it about 3 or 4 times on ER now with different friends.
If you want the achievement, just keep trying on Dead Center. Just because The Passing is short doesn't mean it will be easier to finish.
@gustave154,
It's a whole different beast, but as long as you're with good teammates who stick together and don't shoot each other, it's not so bad. Most of the failed attempts I've been through were due to FF, nothing else. You should definitely try it, but don't PUG it, have 3 other good friends with you. The PUGs I've tried always result in people FFing and killing each other.