I've used a large zone and activated/de-activated the AI depending on whether or not an aircraft of the right type was present. I was building scenarios where medevac helos were landing very close to hot LZs. I gave up on actual foot soldier type infantry; they simply run at each other until they're thoroughly mixed up. I kept light armor engagements sorta dynamic with a mixture of balanced opponents, range, and respawn when dead at a certain percentage. They would turn on and off repeatedly and maintain a fun level of 'splosions, tracers, and fire.