Mission: REDEMPTION
(when you have the Valkyrie missile launcher)
-put the difficulty on recruit
-wait till the choppers are very close together (if you keep moving in small circles they won't be able to kill you on recruit)
-then fire your missile and hit detonate as soon as you are between the choppers to manually detonate it!
The biggest advantage is that you won't have to reload the checkpoint everytime, because now you either destroy both choppers and thus unlock the achievement or you destroy none of them in which case you can try again right away.