In Invasion, there is a red building by the broken bridge.

Go inside and if you have the right building, there should be a cash register and a counter and above a hanging ledge type thing.

Jump on the counter and then carefully onto the cash register by moving slowly to the right. At the edge of it, it is slightly hanging down.

Try jumping on it. After a few times you will hit a very small jump, so you can see above the ledge.

When this happens, crouch and walk on to it and you are on top!

Up there it is very dark so you are hard to see. Also you can shoot people walking by outside the window/door.