How can I mask part of a sprite? Should I use a shader?

Hey there :). I'm trying to find a way that I can create an invisible mask which will (when objects of a certain layer move behind it) make the parts of those objects behind the masks invisible. I wrote out a forum post about the topic which has no responses yet, but it has some images and another description of what I'm trying to achieve.

Here's the post

Anyway, is there a way to do this simply? I have read a bit that I need to use something called Shaders? I looked a bit into this and it didn't seem that promising. Any advice you could give would be very helpful :).