The easiest way to get to get this achievement is to go to a fortress, get caught and have reinforcements come in. Once you see one of many to come helicopters come in, take it down with C4, Explosive Arrow, RPG, M79, and the GL-94. Aim for the central area of the chopper for it to deal damage to the people inside. Should pop first try, I think it's easiest to use the GL-94 or M79.
Hope this guide helped. You can also get helicopters on some campaign missions but if you've beaten the game like me, just go to a fortress.