The main window is roughly 20x12 tiles. Make a location that size, then, make a trigger like this:
QUOTE
Current Player brings Exactly 1 Unit to 'Anywhere'.
Current Player brings Exactly 0 Unit to '20x12'.
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Kill 1 Unit for Current Player at 'Anywhere'.
Where "Unit" is the unit that should die when it leaves the screen.
Damn you Zeratul_101, daaaaaammmmnnnn yooooouuuuuu