[stickynote]

You can use death counters, and if you don't know what they are, I'll explain.
This is the counter:
Players: Whoever you want, but I will use player 1 in this example
Conditions:
-Player 1 has suffered exactly 0 deaths of "Cave-In" (You can use any unit that you don't use in your map, but unused units are the best)
-Player 1 Brings 20 marines to location "raaaaaa was here"
Actions:
-Modify Death Counts for Player 1: Set to 20 for "Cave-In"
-Preserve Trigger
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Cave-Ins can be a representation for marines.
Then, for the computer to send 20 marines later, or whatever retaliation you want after your fight with them is over, you can have:
Players: The Computer, but I will call it Player 2
Condtitions:
-Player 1 has suffered exactly 20 deaths of "Cave-In"
You can have any other condition to have it at a more specific time, like Player 1 brings exactly 0 any unit to location "this is the comp's base"
Actions:
-Modify death counts for Player 1: Set to 0 for "Cave-In" (If you want to reset it."
-Create 20 marines for player 2 at location "Comp Spawn"
-Issue order to all units in location "comp spawn" to attack to location "player 1's base"
-Preserve trigger
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You can fiddle around with this and change the numbers and units.