This Achievement can be obtained in different ways, as each character can die at multiple different points through the story. I went for this Achievement after I finished a play-through and had all characters survive.
To make it easier, make sure you go into settings and turn off Quick Time Event (QTE) timers, unless you want the challenge.
Before we get into the guide I want to quickly let you know what I required to pull this off;
Make sure Conrad gets away on the boat during the thunder QTE.
When using the radio to call for help, DO NOT talk to the military.
Have Brad say we all breathed in the mist to Junior, Causing him to shoot
Have Alex get the distributor cap from Olson.
Make sure Alex stabs the person 4x during the struggle after getting the Cap.
Start by going to;
Play Alone
Scene Select
Load the Chapter called 'Junior' (the third from the last one)
New Save
You'll start off as Brad & Fliss (it could be any combo of Man/Woman I believe) with the rebreather and Olson chasing you (he'll have the distributor cap in hand).
You can fail both the QTEs here dropping the rebreather as it's not needed.
You'll then be in the next area with Junior pointing a gun at you, when in control as Brad (male) make sure he says that you all breathed in a little mist;
Skeptical
Worried
Indignent (sure we all breathed in a little)
This will prompt Junior to shoot Fliss (woman) before Himself, leaving Brad (male).
Next you'll be in a room with Alex and Julia (same thing, could be any combo of man/woman), just follow the path until Alex jumps down a hole, causing Olson to come by and a QTE. pass the QTE here making Olson go away and Julia jump down.
Continue along the path until you reach another QTE where Alex jumps over a hole towards Olson, leaving Julia behind
Continue along the path again until you reach Olson and the rats, making sure to pass the QTE here and collect the distributor cap.
Alex then picks up a knife and heads towards the next room, this will start a QTE fight, just make sure you stab the other thing and not yourself all four times.
Once Alex comes to he'll realize what he did and the game should play out from here.
Make sure you to watch the credits until the end and listen to the curator, The achievement should pop right after they finish and a preview for 'Little Hope' starts.