Destroy the helicopter using frag grenades in 'The Only Easy Day... Was Yesterday'.

Eventually you'll come to a section where enemies repel from the side, and you'll be attacked by a helicopter. Instead of using a rocket to dispose of it, head back into a safe corner and let the helicopter follow you. You'll need to cook grenades and throw them at the helicopter so that they explode midair and damage it. Do this twice in a row to destroy the chopper.



Achievement is worth 20 gamerscore.