Okay.Take your mouse and place the little arrow over the Half Life 2 zombie.Press the right mouse button.Go down to where it says "is immobile" and select "yes".Go down to the bottom and click on the box that saves the changes.
You might also try left clicking on the zombie and using the page up button to place it above the terrain.When your level starts it will drop on the hill and not through it.You might also try hitting the ENTER key when placing your entity so it is on the surface.
You can also try different terrain.Some terrain doesn't work well with FPSC.
While looking at it is nice actually running across it or placing an entity on it could create a problem.It might be best to use hills and such as window dressing.
FPSC is not good at outdoor environments.You can make them but they need to be small and short for the most part.
FPSC's source engine is not designed for outdoor levels.
If this is of no help ot you then maybe uman or someone with extensive knowledge of this can answer your questions more throughly.
Just for the record,I am one of the nicest people on these forums.
FLa
Where you can find my demo:http://www.savefile.com/files/6970524