So a popular grief is to take a hunter or any other large vehicle and ram it into helicopters whilst their trying to pick troops up from base.
Simple fix, put some more H barriers around the landing zone, just too narrow to fit a hunter or other large vehicle through.
Best way of doing this could be, put a timer on the destroy helicopter mission, if its not completed in X time, it takes off and circles where the hanger was for X time, then moves off to loiter around the AO.