To do this it would take quite a few zones, however it could be worth it. Here are the steps:
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Place zones around the border of the map and set them to be off when the game starts
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Have a repeater that activates every few minutes (this is what is slowly advancing the zone)
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Set up a property that is increased when the repeater activates
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Have a checker that checks if the property is a value of one, if so it activates a few of the zones
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If someone steps in the zone have them activate a respawn device or team switcher
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Have the checker continue to check to see if the values are increasing and with each increment more zones should be activated.
Here is the requested screenshot:
Imagine that the zones in that 3x3 grid over there is your map, when the repeater is activated every few minutes it increases a property (not shown in image), also when the repeater activates it triggers a trigger which checks the property value, if the value is one it activates the zones which when activated and a player steps into them the player is knocked out (using the respawn device).
Hope this helped!