im betting your problem is you keep throwing people OVER it? you need to do a ground throw that doesnt pop them up, i had some trouble at first but it's doable lol
I used Lui Kang's flying kick. It knocks them into it. If your throwing them you should be hitting the walls. I assume your throwing them at the wrong wall. It's the wall with sky in the background. Not a door. It's the walls in the background.