During the mission, you will come to a part where you shoot down a helicopter causing it to crash into the train. As soon as this happens, sprint up the left-hand side of the train and keep sprinting until you see some enemies coming up the sides of the train using grapples. Shoot them fast enough to cause their bodies to fall off the side of the train. If you fail, just restart the checkpoint and try it again. You have to be really quick. As soon as you kill 2 of them, you will gain the achievement.
Do you have to knock what looks like all 4 off? Im guessing so...its just that in that first video it looks so much easier wtf. On mine its like they get on SO FAST that I barely can even shoot one of them down let alone all of them.