This mission is all about timing and getting a little bit lucky.

Your options are either to wait for the deal to go awry, which will cost you about 25 seconds, or reach the halfway point on the fire escape stairs to trigger the early shootout. If you do what you see in the video (an early trigger), the mission will become bugged and Packie will die if he gets hit by a single shot. If the attacker reaches him, you have about a fourth of a second to land a shot on the attacker before Packie is killed. The attacker spawns after the first barrel explodes (technically after some of the closest enemies are killed). The barrels will not explode if you aren't zoomed all of the way in when you shoot at them. Watch for all of the red blips to disappear from the map. This is how you know all of the targets are dead, at which point you should get down to the street. Packie's attacker drops a knife when killed. Pick it up if you don't have one at this point. You'll need it later to kill the lawyer during Final Interview. The mission will not pass until Packie's accomplice enters the car. He's killed in the video just after Packie's attacker is killed. Kill him to possibly save a few seconds at most.

Conditions of mission failure: -Wasted -Busted -Packie dies -Shooting before the deal goes down -Dealers get away