NOTE: Please see the edits at the bottom of this for a warning as to actions that can cause you not to get the achievement.
This achievement is a whole lot easier than it sounds but there are a few things to keep in mind. First and foremost - Do not die and do NOT restart the level. If you so much as reload a checkpoint because you missed a collectible the achievement is lost. If you happen to miss something make sure you restart the whole level not just the checkpoint. You can get the achievement by going into the level from the Episode select menu it's just that your playthrough has to be one unbroken run.
Secondly find all of the supply chests in the level. Almost all of them contain a Flare Gun or ammo for it which is invaluable. Lastly there are only two hard parts to this stage - the gate defense and the finale with the chopper - but there are a few tricky spots. So lets go down this one section by section.
The first tricky spot is near the port-a-potties after getting off the street for the first time. Since this is such a close quarters fight it's very easy to be surprised and die here. Either use a flare or just run for the gate in front of you. If you activate the button a light turns on and ends the encounter.
Next is the gate you and Sarah have to defend. It's not that bad at first, what with a heavy Taken, a pair of fast ones and a regular guy who loves to huck axes at your face. Get rid of the fast ones and then just choose your next targets by whoever is more of a threat. When they die a chainsaw Taken comes out with some backup. Now would be the time to use a Flare Gun shot to get rid of everyone and lower the big guys shield.
In the church crypt a pair of large Taken will attack you and the Sheriff. This is a great place to use a flare to keep them away while whittling away their shields with your flashlight. Otherwise they might choose to rush you and that's basically a death sentence down here.
Last but not least is the choppa scene. This is almost laughably easy once you know what you're doing. Forget the red canisters, I tried using them on my Normal playthrough and they didn't help out nearly enough. The following strategy worked just fine on my Nightmare run though - I got this achievement on that run of all things.
When you reach the yard run to the right and gather up the things from the red emergency supply box. Go back towards the entrance and then over to the left emergency supply box before going over to the truck and looting that as well. So long as you stay away from Sarah and Barry the cutscene won't play. Once you've got everything go over to them to start the fight.
As soon as it begins grab the flares and flashbangs off the ground and walk up the steps. Put your back to the chopper and press up against it. Now you just wait.
When you see enemies clustering together fire off a flare. If they get too close to you use a flare to fend them off, possibly followed by a flashbang to show them how you feel about your personal space. While they're at a distance use the flashlight to drain their shields and, if possible, shoot the red canisters to take out any who stay too close to them. The hunting rifle works wonders here especially on Nightmare since it's a one shot kill for most everybody.
The advantage to this strategy is that with your back to the chopper as soon as you can escape all you have to do is drop a flare, turn around and finish the level. It's incredibly easy doing it this way and there's no danger of being cut up, falling off the edge or blowing yourself up accidentally.
Hope this helps!
Edit: As a warning it seems that some people have reported that starting the chapter, shutting it off and then continuing your game counts as reloading so it won't pop. It's best to either finish it once you start the chapter or start the whole thing over if that's the case. It wants you to play from the cutscene with Nightingale until the end of the stage without stopping.
Edit 2: It seems that pausing the game for too long or letting the console go idle / controller shut off, can cause you to lose this achievement. It's possible that the game interprets this as you "shutting down" the game by taking too big a break. This is almost certainly a glitch but as it's not likely to be fixed at this point in time it's best to figure in playing around this and the above issue.