Something you could try is having an invisible Bar across the window. Keep the bar at the "ground level" by using an image point on you players shadow. Then you could have a variable that triggers when a throw happens.
"Compare Throw = true"
"On collision with bar"
Then you just change the sine wave information to make him bounce when he hits the bar.
Alternatively you could just set a Y coordinate that's "Player.Y + number" then do
Compare Throw = True"
"Compare Badyguy.Y = Player.Y + (number)"
I haven't played with Sin waves a lot so you'll have to mess around with the settings to get figure out how to make it bounce, but that's how you'd keep them from falling off.
And if all of that is to complicated then you could give the enemies Physics(instead of bullets or Sine) and toggle it on/off for throws and use the bar as a solid object. Then for throw you use "Force at angle" instead of move at angle , and the bounce happens on its own when it hits the solid bar.