When the first part of the mission has been completed allow Packie and Derrick to leave the bank first. As soon as they step outside make a mad-dash to the alley. In this position you can soften the police and N.O.O.S.E officers so that your accomplices can kill them. Once you have made it past the first police car find cover. You can use the shoot-in-the-leg trick on some of the cops in this mission, including the ones surrounding the police car.
After advancing to the next area where there are a lot of N.O.O.S.E officers, it is important to soften them quickly (use an assault rifle) and don't reduce their health too far, your accomplices will make light work of them once their armour and health is reduced. This is pretty much the technique right through the mission.
When the mission advances to the subway, don't follow Packie and Derrick, use the entrance on the other side of the road. Use the same technique as earlier to get through all the N.O.O.S.E officers. On the train platform take cover behind the third column. Use the shoot-in-the-leg trick on the two police officers opposite that Packie and Derrick (usually) can not hit. Packie will give the instruction to head into the subway tunnel, run into the tunnel and turn to face the advancing N.O.O.S.E officers, soften a couple of them a little and keep moving backwards into the tunnel. This will advance the mission to the final stage.
Ensure that both your accomplices are close all the time (they sometimes get left behind if you go too quickly) Go directly to the get-away vehicle and make sure both accomplices get in before you set off. Lose the wanted level, this can be done during the drive back to the McReary house. If all this has gone to plan you should have completed this mission with zero kills.
There are some bugs and glitches in this mission. I have had the game crash (on several occasions) after leaving the subway (yes, in that final part!). Whatever happens I do not recommend using a taxi to get back to the McReary house.