Find a helicopter or call FLY-555-0100; in order for this 2 work you must have not yet unlocked Alderney OR Algonquin.
Fly over the water to get to the island, and you'll get 6 Stars, and three Annihilators follow you - fly away from them and begin to circle the coast of the island, skipping from one island 2 another if necessary for 5 minutes (fly high so you can't get shot at by cars and boats).
By doing this you'll be untouchable if you keep moving and checking the map every so often.