Marty realizes that the slingshot alone is not enough, but luckily, he has an idea.
He opens the door and climbs onto the truck bed, among the blueberries. Waving his arms vigorously, he acts as a living scarecrow, frighting away the blueberry pelicans. If any of them dare to come closer, he scares them off with the slingshot.
The plan works, and on their second attempt, Marty and Blankie complete the tenth level.