How to get the "Ashley Snaps" trophy in Until Dawn
"Ashley Snaps" is one of many confusing trophies in Until Dawn. There are multiple steps required to set up the needed scenario, and those events span multiple chapters. Missing even one will cancel out your chances of getting this trophy.
The whole point of the "Ashley Snaps" trophy is having Ashley kill Chris by not letting him back inside the lodge. That exact scene occurs in Chapter 8, but the events needed to set it in motion start in Chapter 6.
Since you are killing Chris, this trophy isn't for you if you're trying to keep everyone alive. It will cancel out the "They All Live" trophy. However, this trophy would be a good one to get if you're killing everyone off in one playthrough; "Ashley Snaps" fits nicely with the "This is the End" trophy.
As a note, the choice Chris has to make between killing Ashley and Josh has no impact on this trophy. It does not matter who you choose to kill in Chapter 4.
Step 1. Shoot Ashley
After playing through Chapter 6 for a while, you'll get to the scene where Chris has to choose between shooting himself, Ashley, or doing nothing. Shoot Ashley.
This will make her mad at Chris and set up the events needed for Chapter 8.
Step 2. Play from chapter 6 to 8, don't skip
You cannot skip from Chapter 6 to 8 using chapter select. You have to play from Chapter 6 onwards. Skipping through the chapters will null this trophy.
Shoot all the Wendigos
In Chapter 8, the stranger will take Chris with him to go find Josh. (If you've done everything right up to this point, Chris will leave Ashley without saying a word). When they arrive at the shack, Wendigos will appear and the stranger will die.
Chris cannot die at this stage. He has to make it back to the lodge so Ashley can kill him.
This requires you to shoot all the Wendigos that Chris comes across on his way back. Shooting them will slow the Wendigos down so Chris can make it back to the lodge unscathed.
Watch Chris die
Upon succesful completion of the previous step, Chris will run up to the lodge and beg Ashley to let him in. She doesn't, and Chris winds up getting beheaded by the Wendigos, successfully having Ashley kill Chris.
Step 3. Play through the rest of Chapter 8
Many people think this trophy is glitched, since it doesn't pop right after Chris dies. This is not the case. The trophy will pop after completing the chapter. Do not panic when it doesn't pop right after Chris dies.
This means that right after Ashley flips through the stranger's journal and Sam runs off to find Mike, the trophy will pop.
If it doesn't pop at the end of Chapter 8, you have done something wrong and will need to replay at least Chapter 8.
If you happen to mess up, such as not shooting Ashley or getting Chris killed by the Wendigos before he makes it back to the lodge, quickly quitting to the XMB (the PS4's main menu) and reloading the game will still allow you to get this trophy.
I hope you enjoy your shiny new silver trophy!