I'm not a math expert, so I think my expression is not so good, but you can try this. Find a X coordinate, then Y according to X:
Add a global variable ("randX").
Condition: System -> Every X seconds ->
Action: Set randX to random(0,LayoutWidth)
Action: System -> Create object -> Monster, layer 1, randX, randX<ViewportLeft(0) | randX>ViewportRight(0) ? random(0,LayoutHeight) : choose( random(0,round(ViewportTop(0))), random(round(ViewportBottom(0)),LayoutHeight) ).
You may have to adjust viewport layer numbers, and rounds.
Look at the manual for more details (expressions
, system expressions