Double Whammy: 15G
Destroy both helicopters with one Valkyrie rocket from the deck of the ship.

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In the mission Redemption you will have to destroy to choppers during the mission. For this achievement, you have to take down both with one Valkyrie Rocket. You will have to wait till they are both near each other to do this. If you are on a harder difficulty i suggest hiding behind cover. Make sure you still have an eye on the choppers though. Once they are near each other, aim in and press RT and once you are in the middle of them hold RT to detonate the rocket. It will blow up the two choppers and you will get the achievement.