Here is a couple of ideas a have to make it harder...
Random patrols.
Random spawn.
An extra group of EI that will execute some civilians. The AI will wait 3-5 minutes (from mission start) and then kill the civilians and then either start patrolling the area or get in a vehicle and drive off. the player will have the choice to save the civilians.
50/50 chance a sniper will spawn at 024056, once you have killed the patrol/s. he will make it harder to destroy the light house.