Changing what's needed for capturing a field requires a couple of settings.
* Town percentage down - defaults (I think) to 75%; you can change it to whatever you want, but still need all ack down.
* Maproom hardness - defaults to 10 troops; hardness is in pounds of explosives, troops are "ordinance" like bombs, so as you increase/decrease the hardness you increase/decrease the number of troops required.
Along with this, you can change the "win the war" settings so the map (and thus the arena) won't reset. For FSO events we set it so you'd need 1000% of your bases and 1000% of the other country's bases to reset - which makes it impossible to "win the war".