I can't open Construct 2 now to make an example, but try taking a look at this:
(can't post URL, so take a look at the FAQ, in the link "Metal Slug/Mario type of scrolling")
With this logic, I guess it's easy to fit your style of camera in your game.
I'll try to do an example as soon as I can. I was trying earlier today but I had some problems.
The idea I was trying is that if the player hits the end of the window, you scroll the camera accordingly (with Scroll X and Scroll Y in "System").
Another idea was to create an invisible sprite in the middle of the screen, add the behavior "scroll to" in it and if the player goes beyond the window (viewport), you change the position of the invisible sprite accordingly. It worked, but I couldn't find a way to make it move smoothly (so it's a nice effect). It always moved absolutely, so the camera moved to abruptly.
I hope what I said made sense. I'll try and make a example when I can, but I'm in a hurry right now. So I'm sorry if I wasn't clear enough in my instructions, I wrote it fast.