The combines still aren't as hard as the marines. I don't see how they took over the world in 7 hours.
Have you noticed how the scientists run into the marines and get shot in the face, how they do a bad job of running from aliens, running into trip mines, not being able to dodge enemy fire very well
The security guards are better than them but they still have a habit of dying sometimes
And you wonder how the humans lost to the Combine? They just probably menacingly stared them in the face and the humans went running towards a mine field