I used weapons glitch to get the rocket launcher to destroy one of the boats from far away. Then I get an ammo pick up so I have another rocket. If both boats are destroyed before the objective to take them out is given then the mission will fail. But one can be destroyed early. After that I make my way to the objective trigger. Then I turn around and blow up the other boat. Then I run over a hill to the end ignoring all of the enemies.