I'm planning to film a post-apocalyptic zombie movie soon, and have been wondering how to film a huge mob scene. I'm got about 10-15 actors, and I'd like to film them running down a hillside. Ideally, I'd like to be able to edit the footage and clone the actors so that it would look like dozens of zombies running down the hill. I've seen some tutorials on how to make simple masks and clone one or two people, but that doesn't really apply to what I want to do---I don't think.
I'm using Adobe Premiere Pro CS4, and am somewhere between an intermediate and advanced user.
How can I best achieve the effect I'm looking for?