The "Revenge!" achievement requires the destruction of the persistent Hunter-Chopper encountered during Chapter 4, "Water Hazard". After acquiring the airboat's mounted gun, navigate through the canals until reaching a large, open reservoir. Here is easiest to destroy the chopper, as the Hunter-Chopper will engage aggressively in a large open area. Utilize the airboat's mounted gun to inflict damage while using available cover to evade attacks. Sustained fire will eventually down the chopper, unlocking the achievement.