Simply kill everyone as the chopper comes in. It must be all four or it won't unlock. I can confirm this over testing it with less people both public and private. I recommend boosting it if you are struggling. You could do what I did and find a weak team and let them all reach the end. You should down them beforehand to weaken them and so they use up their revives/heals. I can confirm this can be down while the chopper is coming and while it's on the scene. I started my attack while it was coming killing two of the fireteam and killing the last two members while it was on the scene.