If you've got your player behavior to stay inside the layout along with the scrollto behavior so that you never see outside of the layout, you can spawn the monsters outside of the layout (assuming you don't have your monster with the destroy outside of layout behavior).
If you use rand(-500,layoutwidth+500),rand (-500,layoutheight+500), then check to see if the monster's X is less than 0 OR greater than your layoutwidth and the Y is less than 0 OR greater than your layoutheight. If they aren't, then move them outside of the layout and let 'em go.
If you wanted to get even fancier you could do something like this:
Demo
capx (R117)