I did this in the War Pig mission, Right at the start of the mission, there are a few guys with RPG's and a few helicopters that come into play a short time after that drop enemies off. Simply kill a RPG holder, loot his weapon, aim and fire at a helicopter. Should you miss, or if you die, reload the last checkpoint.
P.s. Only started the channel recently. any constructive points would be appreciated.
Cyber_Stitch