I actually found Blood Harvest to have the easiest first 4 levels though. I went through it multiple times without ever failing any particular segment, while every other campaign usually hits a snag on the third or fourth level.
I spent a ton of time on the BH finale, but it actually isn't that hard if everyone is in the barn. That seemed to be the best spot by far, since all the special infected climb the ladder and stand still for a second giving ample time to kill them. Then the Tank will either go in through the side area or up the ladder, which is easy to see and unload on him. Then just have everyone walk out of the front hole and get him to follow, then climb back up.
I spent a ton of time on the BH finale, but it actually isn't that hard if everyone is in the barn. That seemed to be the best spot by far, since all the special infected climb the ladder and stand still for a second giving ample time to kill them. Then the Tank will either go in through the side area or up the ladder, which is easy to see and unload on him. Then just have everyone walk out of the front hole and get him to follow, then climb back up.